Most people don't gain much from just reading text books. For subjects like English or humanities, analyse some model answers and essays, make sure you know the reading material inside out so you don't waste time looking through in the exams, and try and condense your notes as much as possible into things like mind paps, flash cards, or posters (put them somewhere you'll see every day!).
For maths and sciences, past papers are you're best bet. Go over the bits you got wrong, learn the topics again, wait a few weeks until you don't remember the details of the paper and then you can attempt it again. If you keep a list of the paper codes and your scores over time you should be able to see your scores improve.
If you like revision from books, look into getting some good revision guides. I like the CGP ones but it's a pretty personal choice.