Honestly? Natural intelligence most probably. I did minimal revision, certainly not 8 hours a day like some of the really keen kids on here think you need to do, and my GCSEs turned out pretty good. There isn't really a secret, just try to understand the content, like, don't memorise the whole textbook or something stupid. It doesn't matter anyway, I think everyone has a peak and if you find yourself working really really hard and not achieving what you like, you're either a. doing something fundamentally wrong, or b. simply not naturally talented in that area. Another thing with GCSEs - exam technique can really score you a lot of extra marks, you don't necessarily have to be that good at something, you just have to write what's in the mark scheme, and year to year a lot of questions don't change.
To sum it up in a sentece: work smart, not hard.
This advice is specific to GCSEs by the way, A-levels are different, it's pretty important to revise and revise properly.