Hehe. Personally, I did a very minimum amount of extra work the entire year, basically homework that I couldn't work out of and obviously revised a little for modular tests, but then I simply crammed the night before each exam. Teachers consistently tell students not to do this, but it's worked for me; I got 6 A at A2.
I think it's mainly just about the way you revise and how easy you can memorise things. Of course the subjects make a big impact too, things like Maths, Chem and Bio are real easy to pass as long as you pay some attention in class and understand the theory. Then you only really need to learn the technical stuff the night before.
On the other hand, I'm sure some of my results could have been higher As than they are, if I'd put more effort in and spent less time out drinking with my friends.