Sleep. There have been numerous studies carried out that show that cramming late into the night not only doesn't help, it actively wipes out knowledge. Not to mention the negative effect on your performance of being tired the next day....
I am not a last minute person and had a revision timetable that I stuck to before exams, so I never needed to cam all night anyway :)
Old habits die hard ... I would do a night of cramming. I'm a night owl, so this technique always worked for me. Being much older - I'm still the night owl and although I don't have any tests or exams, I always prepare myself or any important meetings etc ... the night before.
Try to go to sleep then lie away worrying pointlessly even though I have already done heaps of study, then be tired through the exam. Well, that's what I did in my teens. I've matured a bit since then.
I organise my study so that I do not have to be doing it through the evening. I am more likely to wake early and go through the material.
A good night sleep helps me perform at my optimum so that is what I do. I tend to be pretty organised so last minute cramming has not been an issue for me.