If it's a big midterm and if you screwed up on your other midterms, then plan to study 2 days before the midterm. It depends on what your subject is.
If it's more literature related/multiple choice (ie.history) then you could either read all the chapters you need to before hand and then on the night before, go through each chapter and write up a couple of study notes.
However, if it's more math/physics/cs related then you could look over your lecture notes on one day and on the next day(which is the day before the test) repeat some homework problems and practice a bit.
But if you did pretty average/decent on your previous midterms AND you seem to be understanding what's going on in class, then you can just study the night before if we are talking about a math/physics/cs class.
Also, when you are PRACTICING math/physics problems or REVIEWING over history chapters, try to study in groups, BUT not in big groups. 1 or 2 more people is the max. The problem with big groups is that you tend to just goof off and talk about other stuff and waste time.
BTW, the important thing is that the actual READING of the chapters or the actual READING of your lecture notes should be done individually. that makes sure that it gets into your head, lol.