The Art of Revision

Start at least three months before the exams. Prepare a revision plan for each subject, listing the topics and when you will work on them. Keep it flexible, your plans may well have to be modified during your revision.
Write down the key points for each topic, from memory as far as possible then from text books. Find out what you do know and what you don't. Then carefully study the topics you need to and make more summaries. Check your understanding again by trying more exam questions.
Don't just read through your notes, it is too passive. Instead ... do lots of sample exam questions. Know what the examiners look for in your answers. Work through revision guides, they have summaries of the key points, examples of how to answer questions and lots of practice questions with model answers. At this stage you will probably be aiming to maximise your grade.
Short periods of good concentration (up to 30 minutes) are best. Do take regular breaks and exercise, get some oxygen to the brain. You may like to work with a friend - if so, avoid the temptation to just chat! Give yourself little rewards now and then for all that effort.