#How it was recorded in the past

You've said:

How did people record those characters in the past, when communication, research, archival and preservation were presumably difficult feats to achieve?

But actually I'm not clear on what you mean about communication and archiving being difficult feats to achieve. China had printing presses and paper and bound books, and before that bound bamboo scrolls, stored in libraries/collections across the region. You had the Zhou administration sending people throughout the state to do surveys and collect census data. From early on, things like data collection and archiving were not that unheard of.

That said, the way it was done was the way you'd do it today, more or less. For example the Fāngyán (方言) was a very early dictionary of dialectal variation, compiled over 2000 years ago, done as a sort of regional word list (more like what you find in a phrase book than what you'd get from Webster's). People went around the country documenting what people in other places called certain things and eventually it was all compiled into a single text.

A couple centuries later, you get the Shuōwén Jiězì (說文解字), a character dictionary which is one of the earliest (if not the earliest) attempt to describe etymologies of characters. It got plenty wrong, but is still heavily relied on today by the average Chinese speaker as the go-to source for etymology (but really, it got stuff wrong, and the field of Chinese character etymology is still alive and thriving).

#Standardisation and Teaching

Chinese characters have never been all that standardised. You still see a lot of variation that's never really gone away. For example 峰 and 峯, or 群 and 羣, which are for all significant purposes the same thing. There's even more variation which unfortunately isn't really supported by Unicode, so I can't easily type it here to show, but similar sort of stuff.

That said, the most significant form of both standardisation and teaching has to do with the classics. To this day the "Three Character Classic" 三字經 is an early text for children to memorise as a way to learn characters, even though they haven't got a clue what the heck the thing's actually talking about. For those wishing to proceed to the Civil Service Exams, it was the Confucian classics that provided the bulk of the materials and, with that as the ultimate goal in education, it was the same texts which were really the foundation of any learning for the average person. Obviously things like tallying how many bolts of silk you just sold at your shop in the city market don't follow that as closely. But for the most part, what standardisation there is is ultimately the result of convention, as variant forms fall out of fashion and one single form becomes the norm.

There have been specific attempts to standardise characters and get rid of variants. One of the most well known early examples is the creation of Small Seal Script (小篆) under the leadership of the First Emperor of Qin (秦). This was done to unify the written correspondences between the different parts of the country. Obviously it didn't work completely, since variations still survived and new ones developed over time. Still, this was probably the most significant standardisation with the greatest long-term results.

Other later attempts were also made, but still to this day no one single standard exists. Only conventions.

#Decipher ancient documents

One of the secret tricks for reading Classical Chinese is to already know what it says. The language that was in use for writing (Classical Chinese, 文言文) never really reflected the speech of the average person. It was intentionally poetic or obfuscated or some other word that I'm having trouble remembering. Point is, it wasn't something the average Zhou could just pick up and read without having some training. Classical Chinese texts are full of literary allusions, idiomatic speech, references to other documents and that kind of thing, making it really hard to know what's being said if you don't have the background and an idea of the gist before you start. Add to that that there's no punctuation, when the position of the otherwise-absent punctuation can completely shift the meaning. It's a pain. It's a pain today and it was a pain a thousand years ago.

Then add to that the fact that, as you pointed out, the language is constantly changing. There is not one "Classical Chinese". There is a continuum of Classical Chineses that all kinda run together, with different points on the continuum represented by specific now-well-known texts.

The solution to this is the fact that it is a continuum. You had people at point B who could make sense of A, then people at point C later on who could make sense of B were able to then rely on the passed-down knowledge of point B in understanding A, so that we can go back and work out the shifts from point to point in order to understand A from point G or whatever.

It's not the case that you're in 2016CE suddenly discovering a text from 16CE and having nothing else to go on.

#Remembering the Pronunciation

Early (and sometimes modern) dictionaries (using the term very broadly here) of Chinese characters employed what's called the fǎnqiè 反切 system. The idea is that any character's pronunciation can be represented with two other characters. One represents the beginning of the syllable, and the other represents the rhyme and the tone.

An example would be 東 dong which could be spelled with 德 de and 紅hong. Since 德 and 紅 would be widely known characters, even if 東 was not, you'd be able to know that it starts with <d>, ends with <ong> and has a píng 平 tone (since that's what 紅 has).

This is an invaluable feature of older dictionaries not just for people of that time but for much more recent scholars like Bernhard Karlgren (高本漢) who used the system for his early reconstruction. Even today, while we don't rely on this as heavily as we once did, it's still quite useful for determining how pronunciations have changed over time.

So basically you don't need to remember the pronunciation because it's all spelled out for you in the dictionary itself.

#Yup.

Let me know if that all made sense or if you want me to expand on anything.

I'll could probably add sources if people are going to be weird about that, but since nothing I've said is really controversial or not otherwise common knowledge in Sinitic circles, I haven't bothered. Frankly I'm not sure what I'd need to source anyway.