Make revision sheets of the key information in a topic, so that you have to actually process the information out of the texbook/revision guide, rather than just blindly copying. Actively think about the material you're learning, and try to see how it connects to everything else you've learned so far. Check off against the specification to ensure that you've 'got' everything, and then try some past paper questions. Mark them carefully (with the examiner's report), and then go through the questions you've got wrong again with the benefit of having seen the mark scheme, and with your revision sheets alongside you. Anything you're missing, add to the sheets!