For sciences, this works, but i couldnt say for other subjects:
Make sure you know the syllabus and everything on it, and make clear, consise notes which are compressed, with little waffle and no questions/answers in. you should be able to just look for a few minutes when you have the time, like on the bus and also for longer too if you want.
And the major thing to do: ALL THE PRACTICE EXAMS YOU CAN FIND, this includes non board papers, like delphis papers for maths, if you can get hold of them, as they are really useful.
EDIT: if you find a part you dont fully understand, work at it till you do get it, then write down clearly how you understood it, so if you forget why it made sense, you can easily see why again.