There is no substitute for starting early with revision.

You need to give yourself enough time to review everything that you have studied and make sure that you understand it (or to read round the subject or ask for help if you are struggling). Last minute cramming is much less productive.

Ideally, review each subject as you go, and make sure that you understand it fully as this will make revision much easier. Ultimately, the best tip is to study hard and know your subject, and starting early is the best way to achieve this.