A couple factors that helped Britain win.

Radar/observers - Using radar they could detect incoming waves of German aircraft and direct their fighters to intercept them.

Fighter command - An efficient command structure used by the British that enabled them to quickly decide who to send up and where to send them.

Location - German fighters could stay on location and dogfight with British fighters for only a few minutes before having to head back to Germany due to fuel constraints.

Location - British fighter pilots who ditched out of their aircraft were above friendly territory and able to be taken back to their bases and given another plane (assuming uninjured). German pilots were captured, unless they bailed out over the ocean and even then there is no guarantee of rescue. Whenever a plane went down the Germans effectively lost a pilot even if they bailed successfully.

Location - Damaged British fighters were never more than a few minutes away from a friendly airfield. German aircraft would need to fly back across the channel to their airfields in France roughly 30 minutes away.

Static defense - British air defenses and raid balloons took down their fair share of German fighters.

Personnel - If you lose a fighter you lose one man. If you lose a bomber you lose 2 - 5 men. This is not to say that Britain was not using bombers to strike back at the Germans, but the majority of the battle was British fighters against German airgroups composed of fighters and bombers.

Morale - The British were fighting over home country for their home country. They knew when and where the Germans were coming and had relatively short flight times to interception. The Germans had to fly over the channel, strike their targets, and fly back over the the channel. The Germans coined a term "channel sickness" associated with flying across the channel and back again. This was also the first battle where the Luftwaffe could not establish air superiority.

Equipment - I have read anecdotal reports from WWII fighter pilots that the Hurricane and BF-109, while having distinct differences in performance were more or less equal in regards to the superior pilot winning the dogfight. The German bombers on the other hand were all twin-engined medium bombers, with the exception of the Stuka. They did not have the defensive armaments that Allied heavy bombers of the later war years had and thus were incredibly vulnerable without fighter escort.

Strategy - Germany stopped its attacks on airfields in favor of terror bombing. There is speculation that if Germany had simply continued its attacks on the airfields of the British forces for a couple more weeks they would have been able to establish air superiority as the airfield attacks were destroying more aircraft than Britain could produce.

source - http://www.ibiblio.org/hyperwar/UN/UK/UK-Defence-UK/UK-DefenseOfUK-9.html

source - http://www.historylearningsite.co.uk/battle_of_britain.htm

In no particular order:

The air defence system created by Major General Ashmore in WW1

This was the basic system of detection, communication and control which was improved and augmented by radar in WW2.

UK Air Ministry

Air power doctrine in the 1930s was that "the bomber will always get through". Air force budgets were accordingly largely spent on the bomber arm. In the mid 1930s the Air Ministry started to give some priority to fighter defence, mostly because fighters were cheaper than bombers.

Radar combined with the UK coastline

Integrating radar with the air defence system was a force multiplier. It allowed the sector controllers to intercept raids before bombing, to match raids with appropriate response, and to avoid RAF fighters being destroyed on the ground.

The UK coastline made it easier to detect enemy raids.

RAF: Personnel - especially Air Chief Marshal Hugh Dowding and (at the time) Air Vice Marshal Keith Park.

Dowding had an extraordinarily broad grasp of the whole range of air defence from command and control through to equipment. For example this led him to reject the unsatisfactory Boulton Paul Defiant. He integrated radar into the command and control system, and made the system much more resilient.

Park had an unparalleled grasp of fighter tactics. In later simulations where people had days to second guess decisions Park made in minutes, Park's calls could rarely be improved.

RAF: Equipment

The investigative and experimental approach to fighter aircraft and armament by RAF personnel. For example then Squadron Leader Ralph Sorley calculated and demonstrated that a fighter needed to be fitted with at least 8 machine guns for the average pilot to have a reasonable chance of shooting down a bomber.

RAF: aircrew from overseas

The RAF had a large number of aircrew from the Dominions and from occupied Europe. For example 145 Polish fighter pilots served in the RAF during the Battle of Britain. This gave the RAF a large reserve of skilled, trained and motivated aircrew.

Luftwaffe: lack of clear aims

Were they trying to destroy fighter command? To block British ports? To damage the British aircraft industry? To facilitate an invasion? Or all of these at once? No-one was quite sure, so there was a lack of focus.

Luftwaffe: poor intelligence

Joseph "Beppo" Scmidt of Luftwaffe Abteilung 5 made a good estimate of the RAF order of battle, and the location of key aircraft factories. But he got the production rate fatally wrong, which led to the Luftwaffe overestimating their effectiveness in a very damaging manner.

He also failed to understand the nature of the RAF command and control system, incorrectly believing that fighter squadrons were tied to their sector controllers.

Luftwaffe: lack of depth

The Luftwaffe had few reserves of either men or materiel. The level of attrition suffered during the Battle was unsustainable, especially in the crucial single seat fighters. This was made worse by the loss of all Luftwaffe aircrew shot down over the UK.

Luftwaffe: inadequate tactics

Perhaps expecting a lot more effective light flak over RAF airfields, the Luftwaffe generally avoided low level attacks which would have taken them under the radar. Nor did they make much effort to degrade the radar stations.

The Luftwaffe had no monopoly on poor tactics: the RAF had ineffective formation and fighting tactics at the beginning, and slowly learned by copying the Luftwaffe.

Thank you everyone!

My understanding is that radar gave the RAF a sizable advantage, enabling them to be more effective with fewer planes.