1. QUESTIONS / PAST PAPERS / PRACTICE ESSAYS!!!!!!!!!!!!!!!!
I can't stress this enough. There isn't any point in reading a textbook and forgetting everything - you need to make detailed notes to learn it initially (or else you'll have massive gaps and won't be able to relate things), but the human brain learns fundamentally by being tested. You need to make it do some work.
With sciences, mark the questions yourself, as you learn more that way. However, with essays ask your teacher to mark them (they may say no, but I'm sure they'll do a few over your course). You can't mark these yourself, obviously.
2. MEMORISATION.
With sciences in particular (though it's very useful for other subjects), make tables of key areas and memorise them
about 1 day before the exam, and then again just before. Examples would be a map of organic reactions, equations and conditions in chemistry, or a table or chemical testing results (flame test etc). These aren't the general picture, which you should know, they are the details that will get you an A*. You'll know what sorts of questions to expect from your past papers, and they will require that level of detail.
3. General Picture
Very important, but perhaps one of the easiest to learn. Start with detailed notes - you don't have to re-read them, you learn by writing them, and that will be consolidated by practice questions. Read around the subject.
Anyway, sorry if that was massively patronising, but I genuinely think those are the main three things you need. I can't think of any tips or tricks, apart from the vital importance of 1. and 2., but I'll let you know if I do. May be helpful to say what subjects you're doing
I got: