I do edexcel maths, and i got an A (90% UMS) at AS. what i did was go through the book, don't go through every question, but read the examples in the edexcel books, THEY ARE THE BEST BOOKS I HAVE EVER USED. Do the exam style questions at the end, it's in the mixed exercise. Ask your school for the CD that comes along with the book or order it yourself, since there's only answers for the questions, no actual step by step break down. Don't do LOTS of past papers, do past papers, but take time to understand where you went wrong, it's not about how much you do, but rather how you do it. I only did about 6 past papers for Core 2, and got 94 UMS.
I do AQA chemistry, and biology.
For AS biology, it's all about the your exam technique, i was rather crap at this, i got a B in unit 1, i then resat it and got an A. Make sure you highlight words, and learn the keywords, don't just regurgitate facts, that will not get you anywhere in a level. For bio, i'd say do past papers, the more you do, the better, because bio is all about application questions, especially A2. AQA have lots of past papers that you can find by asking your teachers. I used the CGP book for AS bio, and unit 4 bio, because the cgp books are really good for a level biology, apart from unit 5.
For chemistry, it's more about understanding the science, like maths, you have to understand why it happens. So, rather than just doing past papers, make sure you understand the content. Chemistry was my best subject at AS, even better than maths, lol. And what i did was to go through the specification, make sure you understand every little bit, and always do the summary questions in your book (AQA).
Hope this helps, i have core 4 edexcel tomorrow, and it's a replacement paper, yay!!!! (Sarcasm)