This post could be as short or long as I like because this is an absolutely massive topic especially if we start including every nation and how they worked independently. I will, however, be focusing exclusively on the British as they are the ones I have the most source material from. I will also generally be focusing on trench life rather than trench assault tactics, as the former stayed relatively similar throughout the war and the latter went through much innovation. Specifically,

How safe were the trenches? What were the conditions? How did they keep moral up?

Luckily for you (and unlucky for me since I have to manually transcribe it) I have the daily schedule of one Captain Geoffrey Bowen with the 2nd Battalion Lancashire Fusiliers on September 3rd, 1917. I'll be giving some context in brackets throughout for reading sake:

8.pm. Started [wake up]
9.30 p.m. Arrived. [at trench]
11 p.m. Company arrived.
11 p.m.-3 a.m. Round the line [ie: checking on men, checking positions, maintaining quality control across the line]
3.15 a.m.-4.15 a.m. Sleep
4.15 a.m.-6.am. Stand to. [Night watch, essentially]
6 a.m.-6.30 Reports [from lower level officers]
6.30a.m.-9. Sleep
9 a.m.-9.30 Breakfast: bacon, eggs, tinned sausage
9.30 a.m.-10.10 Round the line
10.10 a.m.-12. Reports, etc.
12.30 p.m. Lunch: Steak, potatoes, beans, sweet omelette
1.45 p.m.-2.15. Daylight patrol.
2.15 p.m-2.30. Sleep.
2.30 p.m.-3.40. Gup [gossip, idle chat] with the C.O. 
4 p.m. Tea, bread, jam.
4.30 p.m.-4.35. Sleep.
4.35 p.m.-5.10. Entertain 'Bowes' 
5.10 p.m.-5.15. Sleep.
5.15 p.m.-5.25. Trench Mortar Officer reports.
5.25 p.m.-6.15. Sleep
6.15 p.m.-6.35. Entertain Brain and Padre [Chaplains, implied work on mental and religious health]
6.35 p.m.-7.30. Sleep.
7.30 p.m.-8. Round the line
8 p.m.-8.15. Dinner: steak, potatoes, tinned fruit and custard.
8.15 p.m.-9. Round the line
11.30 p.m.-12.30 a.m. Sleep.
12.30-2.30 a.m. Intensive sniping [under fire]
2.30-5 a.m. Sleep.

It's not nearly as dramatic as you may think. The unfortunate truth for Hollywood is that most of WWI was sitting around improving defenses and doing basically nothing. The conditions were horrific the entire time for most parts but you were not constantly getting out of trenches and charging enemies most of the time. One of the biggest jobs of men on the front is to constantly check, repair and lay down barbed wire outside of their trenches. This was generally done at night for obvious reasons and generally required hundreds of men to cover the workers doing this. At first they had to use mallets and even if they tried to muffle the sound by putting sandbags between the mallet and the stake to hold the barbed wire down, it was still noisy business. This brought the attention of many snipers. Eventually a corkscrew type of device would be universalized which would allow men to 'screw' the stake into the ground silently.

However the amount of fighting and what fighting you got depended on your sector. There were generally two types, quiet and loud sectors. Loud sectors were ones where the trenches were extremely close to the Germans -- at times less than 25 yards away but usually no further than 100-200 yards away. You are in constant threat of rifle fire but not so much artillery lest each side hits their own men. So your entire existence is painted by avoiding snipers, being under sniper fire, and having bursts of machine gun fired in your general direction in your daily life. The quiet sectors were generally very different. You could easily be 600-800 yards away from the other trench and both sides adopted a 'live and let live' philosophy and your greatest threat would be random artillery barrages from miles away. Capt. Dugdale described the experience:

Time passed very peacefully, as the Germans were very quiet. My battalion snipers had the time of their lives; never before had they been given such targets. We literally kept a game book of hits for hte first three days; after that the Germans did not show themselves so much; also they started to retaliate.

Wiring was carried out nearly every night, but not in the style we were accustomed to in the days of the SOmme. Our men did not creep through the wire carrying coils of wire, stakes, etc.; instead, a general service wagon was driven into No Man's Land with the materials on board, which were dumped out when required. At first we expected bursts of machine gun fire every minute, but nothing happened. It must have become a well-established custom, as the enemy did the same thing themselves; we did not interfere.

Nonetheless in the general, the Germans were very keen on disrupting workers parties; particularly with machine guns and offensive patrols. The need for quiet was imperative but not always followed by the more reckless green horns. One account by Henry Gregory describes a particularly loud worker party shouting orders and joking with each other while his company was covering their duties. After about 30 minutes of it the Germans (who were previously pretty quiet) got fed up and unleashed a massive mortar barrage and machine gun attack on the position, killing dozens of men who had no reason to.

Conditions in the trenches were universally pretty shit however. That is one universal thing that can be applied. Many trenches had water up the knees of men and you would have to wade around in this grungy, dirty mud water all day and everything you had would be almost constantly wet. When digging new trenches it was not uncommon to get a sudden and sharp scent of a dead body lying there for weeks or months as you pierced his flesh in the dirt, especially in when repairing trenches taken over from the enemy after large artillery barrages. Everything, once you got up to the front, had to be carried by hand for obvious reasons. Usually in the dark. In knee to waist high water. While being shot at by snipers consistently. You can imagine the frustration and how it could wear on a man.

That's really what made the war so horrible. You didn't attack all that much if you were a soldier but your life was still a miserable hellhole. You sat in a crappy trench while being shot on constantly by snipers or being bombarded constantly by artillery depending on where you were -- if you were in a perfect spot both at once! You were constantly slightly hungry because of poor rations and if someone slipped and dropped a box of steak in water they were done for and you had just go without. Something that happened enough for men to justify writing about it as a part of their experience. However, for all that, the actual combat was pretty minimal and dare I say cushy, especially for quiet sectors. Your duties if you were a rifleman were essentially forward patrols from time to time and covering worker parties (usually the two duties were combined) which was a dangerous job but not really an all out attack and otherwise maintaining the trench system through constant labor. If you were a machine gunner or a sniper your life was essentially to sit in one spot for hours and harass the enemy and discourage them from performing their own maintenance or making them do it under great duress. And if you were an officer your job was basically to walk around and make sure everyone was doing their job correctly.

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How did charges actually work?

Now that I'm [mostly] through with this post I'd like to talk briefly about how trench warfare worked. At first it was a crude type of deal, the Generals were literally learning on the fly. The original tactic through 1915 and 1916 was essentially bombard the enemy trench with so much firepower that they couldn't possibly survive and then mop up the rest with your infantry. This was basically what The Battle of Somme was supposed to be -- one of the biggest failures of the war where the British men advancing quickly found that the artillery barrage did nothing to the enemy barbed wire and the Germans just huddled up underground ,waited for the barrage to stop, and then just manned their machine guns again once the assault started. Things like the creeping barrage were developed as well where basically the artillery would 'creep' to the German trench as the infantry marched behind it. The idea was that the artillery would hit the trench and within seconds be struck by British and French troops in the immediate aftermath.

Again, the issue was coming with that all out artillery barrages where the men were marching was a horrible strategy. This is most demonstrated at the Battle of Passchendaele, or the Third Battle of Ypres, where the British attempted to break out of the Ypres salient in Belgium by taking surrounding ridges. The British absolutely unloaded artillery on these positions and when the men went into battle the ground was so utterly destroyed the entire battlefield was composed of flooded craters. The men were literally getting stuck in the mud and could barely move and they were cut down endlessly. The battle was only a half success, only capturing a few ridges with egregious casualties no one predicted even at this stage in the war.

/u/jonewer mentions that this was an artillery war and in many ways it was, but it was a war that was won in the development of infantry doctrine. What generals realized by 1918 was that artillery can not win this war. It could not single handedly destroy the enemy like they believed and the principle of combined arms was developed. Combined arms stated that every component of the army must be used together in equal parts to support each other and win the battle and that's precisely what happened. Artillery was used in short, concentrated bursts and barrages not meant to obliterate the enemy defense but just shock them and generally create temporary weak points. Infantry stopped being a force that charged into trenches trying to overwhelm a position "shattered" by artillery but rather began doing something we are more familiar with -- squad based infiltration tactics. Small squads of men would independently infiltrate enemy weak points, neutralize key points and create an open path for friendly mortars and flamethrowers to move in to create a combined mortar, machine gun and flamethrower assault on the more fortified positions with the infiltrated elite troops attacking from all sides inside the trench as well. Combined with aerial reconnaissance, armor to shield advancing infantry, and short but sweet 'hurricane' barrages trenches became all but a stepping stone in the March 1918 offensive by the Germans and then for the Allies in the Hundred Days counter-offensive in August which ended the war.

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Notes:

Holmes, Richard, "Tommy: The British Soldier on the Western Front "

Simpson, Andy "Hot Blood & Cold Steel: Life and Death in the Trenches of the First World War"

Conditions in WWI trenches were generally, as popularly described, awful. However, the trench system changed as the war went on, from smaller defensive and communication trenches of 1914 to the vast defenses of the [such as those seen on the Hindenburg Line] (http://www.oocities.org/bunker1914/Frontalltag_Verdun_Schuetzengraben5.jpg). Therefore, combat tactics and conditions changed as trench warfare evolved.

In the early trenches, combat was very dangerous. Tactics quickly became obsolete, where troops would [line up] (http://medias.photodeck.com/048c085e-3c72-11e0-bf6c-5533370da117/001087_xgaplus.jpg) [shoulder to shoulder] (http://cdn.theatlantic.com/static/mt/assets/international/German%20trenches%20for%20article.jpg) to engage the opposing forces with their small arms. This was extremely risky due to the use of artillery fire from far behind the battle lines, which could easily cause massive casualties in the rather open trenches with little protection from barrages. Therefore, [deeper and more permanent trenches] (http://i.dailymail.co.uk/i/pix/2012/11/01/article-2226235-0F7F8D0300000578-696_964x682.jpg) were dug, with barbed wire becoming ever-present to help defend the lines from charges. More destructive weapons became a mainstay of trench warfare, like [machine] (http://upload.wikimedia.org/wikipedia/commons/c/c9/Vickers_IWW.jpg) [guns] (http://eng4620borthwick.files.wordpress.com/2012/05/world_war_one.jpg) and mortars, increasing the lethality of the trenches and prompting more complex systems to be developed, which would like [something] (http://img88.imageshack.us/img88/4092/trench.jpg) like [this] (http://dianaoverbey.files.wordpress.com/2012/01/layout.jpg).

Since these emplacements became virtually permanent, unsanitary conditions were prevalent. The trenches were stagnate positions in a war zone after all, so the rats, disease, and mud that help characterize WWI fighting today began to infest. Medical services were rather poor as well, later standards, so the wounded faced both poor treatment and secondary ailments such as gangrene. Various parasites and fungal infections loomed over any soldier in the trenches as well. However, the average serviceman on the front line would rarely spend a relatively small percentage of his time in the trenches themselves, since most powers could afford an earnest rotation policy. Even so, [97 percent of those on the front contracted lice, and 15% were affected by lice-spread trench fever.] (http://www.firstworldwar.com/atoz/bodylice.htm). Downtime was possible, however, with more leisurely activities taking place during the day, when snipers and artillery kept most inside the defenses.

Common weapons were the infantryman's bolt-action rifle, such as the Lee Enfield and Mauser rifles. Hand grenades also became a part of the soldier's equipment. Machine-guns, like the Vickers and the MG 08, were the cornerstones of defense. WWI also saw the development of tank warfare (first used in the Somme offensive) and was a driving force in air superiority development. Airplanes were used mainly as observational utilities, (largely ineffective) bombers, and more effective fighters that would wrest for control of the airspace over the trenches. Howitzers, too, saw great development, being vital to the artillery barrages that would fire conventional shells and poison gas on enemy positions.

Not much of an answer really and its an area I'm trying to explore myself.

One thing thats important to realise is that this was an artillery war - most casuallties were caused by artillery and the efficacy of artillery often determined the success of the attack. As Middlebrook points out, many of the casualties on the first day of the Somme were caused by the German barrage falling in no mans land rather than machine guns.

Junger repeatdely describes trench fighting as grenade-based. In effect, stroming trenches often meant a bombing contest.

The field manual I have linked to corroborates this and shows that (British) platoons would have seperate, specialised, bombing and rifle bombing sections, as well as rifle and lewis gun sections. Due to experience in 1916, British platoons would have desginated mopper-uppers who would clear captured trenches to prevent the rest of the platoon being fired on from behind as it advanced. Note that from the manual, there are references to fire and manouerve tactics with some detachments obtaining fire superiority while other detachments attempt to flank. From this we can deduce that the tatics by 1917 were becoming quite sophisticated.

Hope this helps!

https://archive.org/details/instructionsfort00washrich note this is a British manual reprinted in the US

Ernst Junger - Storm of Steel

The first day of the somme - Martin Middlebrook

White Heat - John Terraine

There's a bunch of different facets to WWI trench warfare, but I'll try to briefly touch on most of them. I'll focus mostly on the Western Front, since I'm most familiar with that portion of the war. But it's worth keeping Gallipoli, the Italian Front, and the Eastern Front in mind.

Sniping certainly deserves some mention. At the start of the war, the Germans demonstrated a clear superiority at sniper warfare. At that point, Germany produced more rifle optics than any other nation in the world and by 1914, they had specially-trained scharfeschützen ("sharpshooters") ready for war. Plus, large numbers of Germans had experience hunting and could apply this knowledge to sniping. As a result, German snipers were a deadly adversary for Allied soldiers. British lieutenant S. Shingleton recalled an incident in February 1915 where "to be seen standing in the open meant instant death from a sniper's bullet. It was even dangerous to wear a luminous wrist watch, for the light from same could be see a long way off in the pitch darkness; a few snipers' bullets had whistled past and the RE [Royal Engineers] officer in charge suggested that my luminous watch was the cause whereupon I promptly put it in my pocket." In 1915, the average BEF battalion on a quiet part of the line could lose 12-18 men a day to sniper fire. ANZAC and British troops in Gallipoli sustained similar loss rates from Turkish snipers. Capable snipers shooting at ranges of 200m-300m could easily get first-round hits on any soldier foolish enough to expose himself.

In order to compete with German marksmen, the British rushed to develop their own snipers. By 1915, the British had established a 17-day sniper training program in France an began to turn out competent snipers of their own. Some units, like the 'ghillies' (Scottish gamekeeprs) of the famous Lovat Scouts became legendary for their skill at observation, camouflage, and ,marksmanship.

Counter-sniping (i.e. sniper vs. sniper duels) became an important mission for marksmen. But snipers were exceptionally difficult to kill, in large party because they were so difficult to find. The battle-scarred Western Front offered plenty of hiding places and snipers generally wore some kind of homemade camouflage. Plus, snipers often armored their positions. German snipers often used metal masks and mobile steel parapets to protect their position from counter-snipers.

Trench raiding was a common occurrence during the war. In order to harass enemy forces, capture prisoners, and gather intelligence, WWI troops would occasionally stage nighttime raids on enemy trenches. In most cases, soldiers left behind their bulky rifles and bayonets and preferred to bring along a wicked-looking collection of clubs, knives, sharpened shovels, pistols (some Webley .455s even had bayonets!), and hand grenades (also known as "bombs". Small parties, usually no more than ten men would sneak across no-man's land, cutting or climbing under barbed wire as they went. If all went well, they'd make it into the enemy trench unnoticed. If things went badly, they'd be spotted, illuminated with flares, and cut to pieces by enemy fire. One they were in enemy trenches, they'd set about their mission. This might be quietly taking a prisoner and slipping out. Or it might be causing as much ruckus and damage as possible before slipping out again. Trench raiding was very dangerous, very bloody, very personal work. One historian observed that trench raiding "required nerves of steel and split-second reactions when coming fact to face with the enemy."

Later in the war, German stosstruppen and Italian arditi units began to develop special close assault tactics that relied on surprise, speed, and violence of action to punch holes in enemy weak points and establish breakthroughs that friendly troops could then exploit. These troops pioneers the use of submachine guns and portable flamethrowers.

Much has been made of the fact that attacking WWI troops "slowly walked through no-man's land" into the teeth of machine gun fire. The 1st Day of the Somme is often cited as an example of this. There is a certain element of truth to this. At least in the early war, troops involved in "big pushes" had to advance across open ground under fire to attack enemy positions. But the speed of their advance was a product of battlefield conditions rather than incompetence. Firstly, attacking troops weren't carrying full kit; but what they did carry was still an encumbrance. Webbing, ammunition, rifle, bayonet, helmet, backpack, rations, etc. all added to a soldiers burden. Secondly, battlefield conditions made it extremely difficult to move quickly. In many portions of the Western Front, no-man's land had become a blasted morass of mud and craters. To make matters worse, barbed wire in front of positions like the Hindenburg Line could be arranged in belts up to a 100m deep. As at the Somme, preparatory bombardment often didn't breach the wire, forcing attacking troops to pick their way forward under fire. Thirdly, no-man's land could be fairly large in some portions of the line. Sprinting 500m through mud and shell craters simply wasn't always an option.

WWI provoked a rapid acceleration in artillery technology and tactics. Technologically, artillery diversified greatly. Everything from trench mortars to the infamous "Paris Gun" were used in action by WWI's combatants. As other posters have noted, artillery tactics also became more sophisticated as the war progressed as armies tried aerial spotting, gas warfare walking barrages, pre-registered fires, "hurricane barrages," pre-registered fires, and various other schemes to sue artillery more effectively. In some battles (e.g. Verdun) artillery caused 70-80 percent of overall casualties. Artillery bombardment was a constant threat for soldiers in the trenches and could be terrifying experience, even if soldiers had well-prepared defensive positions.

*Robert Maze, The Webely Service Revolver

*Martin Pegler and Ramiro Bujeiro, The Military Sniper Since 1914

*Martin Pegler, Sniper Rifles