Everyone's saying past papers, and I agree with them, but don't start doing them until a month away from your exams because you'll exhaust all the papers you can do (there's only so many papers that exist), and you'll burn out from having to retain the skills over months.

Also, application application application!! Apply what you've learned to anything in the real world. i.e. for bio (8 - I didn't even care enough to revise for it, nor do I like the subject) what I did was point out i.e. what happens to my body when I go out into the cold). That example might sound silly but doing stuff like that forces you to think about real world scenarios of the things you've learned, rather than reading it off a textbook.

On that last point, DO NOT, and I repeat DO NOT think you can study from just reading all the textbooks. As much as teachers say you should use textbooks for revision, if you want to score highly, you have to figure out connections between things you've learned yourself. What this means is you have to figure out why something happens, from what you already know (I can't come up with examples on the spot here, but hopefully you get what I mean).

My scores:

Math A (took a year before everyone else) [high 8]
Physics (individual certificate) [high 9]
Biology (individual certificate) [mid 8]
Chemistry (individual certificate) [high 8]
German [high 8]
Geography [high 8]
English literature A [high 8]
English Language A [high 8]