It may sound obvious and/or stupid, but quality over quantity. Doing 2-3 hours of focused study a day is better than doing 7-8 hours you don't really want to do. It won't stay in your head if you force it. Study when you want to, not when you feel that you should...to a point, anyway. And don't pull all nighters unless you're inclined to naturally. Cramming does not work. All it does is apply pressure to learn something that will leak right out of your head when it matters.
On a minor note, for exams, I'd recommend starting a month in advance and just doing a little bit every day, rather than 8+ hours a week before. Give yourself time to learn something rather than just expecting yourself to learn it on short notice. We all function slightly better under a little bit of pressure, but exams are not often just a little bit of pressure, and there's never just one =x
One thing I found really helpful was tiny notebooks/study cards, though. Write stuff out and read it a few times a day - before you go to bed, before/after you get out of the shower, when you're eating, travelling; whatever.
This is also incredibly vague, but find out how you learn best and base your study around that. A lot of things like mind maps etc. are worthless to some people because they can't visualise them, and people are always recommending stuff like that. If it helps you, then great, go with it. But if you learn better by writing out notes, or by listening to audio, or visualising things, then go with that and only that. Don't mix and match study techniques when some are more or less effective than others. Try it, but don't stick with it if it doesn't produce results.
...and obviously, focus on areas of a subject where you're weakest. If you know it, you know it. Check to make sure you do every once in a while, but the point is to learn new information more than it is to reinforce old. A lot of people I know spent all their time consolidating and never bothered to try and learn the things they found difficult.