Might not be as useful for Chemistry, but with Psychology I get a few different textbooks and for each topic take the best/preferred condensed information and rewrite it all in my own words, so it's easier for me to understand, remember and replicate.
Actually worked better than expected, had a mock exam today and managed to completely remember 2 full essays after only 2 days working on them!
In Maths I tend to just do past paper after past paper, like others have said, I think Khan academy is highly rated although I don't use it myself. When you're doing past papers for the first time, don't set a time limit, or restrict yourself. Use all your notes, mark schemes, google answers etc. so you fully know and understand. Then as you re-do past papers you can set yourself a time limit, and use no notes at all and you'll definitely find that you've remembered loads of it.
(Flashcards are quite good too, closer to exam time though.)