For o-chem and anything quantitative: practice problems, over and over again. Go over worksheets and other homework assignments multiple times until I know exactly what I'm doing and why. I don't read the textbook too much, except for sections where I don't get a certain method or mechanism.
For bio/non-quantitative: I take a lot of notes, and tend to write things out in a way that makes senes to me, and I'll review my notes afterwards. I'm known for making "study packets" for all exams, which is tedious but helpful.
Also, recording lectures is helpful for me; I invested in one of those pocket-sized recorders that lets you convert recordings into mp3 format, and I'll upload all lectures onto my laptop and listen to them again in the week or so before an exam, and go over my notes at the same time. If I hear something in the recording that I didn't have in my notes, I'll add it in.