Work hard, not long.
I revised Chemistry's second A2 module the morning before the exam and got in the high 90's (which was surprising, since previously I was a simply a solid A student, but no higher).
If you focus 100%, you can achieve a more in less time than those who spend weeks and months, revising for many hours a day.
Maybe it is just because my learning style is different, but I take the attitude that there isn't actually that much in the syllabus (I can only speak for the Sciences), so if you are taking more than a day or two to cover it then you are doing something wrong.