I got a 53% on my calculus final. NO idea why. I'm done studying lots for tests. It seems when I just kinda glance over the material, I do really well (93% on my second calculus midterm, for example; my highest psychology scores were from just reading once through the book) but when I study all day for something and practice, I do worse than I should. This test, and my lowest psychology scores are examples of that. :/
I really don't know what to tell you; it's hard to know how much studying to do, and it varies from person to person. I will say that stressing out about the exam will cause you to do worse, especially if you eschew food and sleep in order to study/cram. I can remember one of my organic chemistry exams. I studied the hell out of the material, but I had done so at the sake of getting a decent meal down me and a good night's rest. I just couldn't think at all during the exam.
One other practice I've never really understood is the all-nighter to study. I don't know when or where the idea that an all-nighter before a big exam was a good thing to do, but I do my best to discourage the practice. When cramming, one topic leads to another, and soon it feels like there's too much information to learn in such a short time. Yes, it's a lot of information, but just take a step back and realize that most of it you've already learned from class and homeworks. Then, get some sleep.