What I would add to these good suggestions:
Keep your workload manageable. Don't take more classes than you have time to do a good job with. If you think taking fewer classes at a time slows down your graduation, remember that flunking classes will have that effect too.
When possible, take classes that you're interested in, and that you have an aptitude for. Not all people are equally suited to physical sciences, to math and economics, etc. Within the bounds of the requirements, you should gravitate toward a program of study that fits your talents.
In any given subject, when you're studying for a test, start by looking at the material that you feel most comfortable with. Reinforce your comprehension of that, and use that as a point from which to venture into the harder parts. Related to that, look for ways that different themes fit together, and think more in terms of comprehension than of memorization.
If part of what you're doing involves math problems or anything similar, when you find yourself getting a particular kind of question right, don't make the mistake of leaving it and moving on. Reinforce it by doing a few more, and be sure to come back to it soon after. Big point: take time to reinforce what you already feel you understand.
Try to relax when you take a test, and try to think of the test as having a continuity with your studying. The story is told of a man in the 19th century who, because of his love for theatre, was given a part in a play where, at the very start, he was to run onstage and say "Hark, I hear the cannons roar." All day long he was boasting to friends of this honor he was getting, and was annoying them by saying "Hark, I hear the cannons roar, Hark, I hear the cannons roar," constantly. Well, he got to the magic moment. He was onstage, there was an audience full of people looking at him, and he was frozen stiff with terror. Then he heard a booming sound from offstage, and he cried out, "What the hell was that?!" Don't be like that guy when you take a test.
Make choices about use of time. There should, of course, be evenings when you're out with friends, but there should also be evenings when you're home studying. Work, club, and extracurriculuar activities also have to be balanced. You have to make sure you have enough time for what you have to do.
Don't use any psychotropic drug stronger than a cup of coffee to stay awake. Don't use No-Doze pills under any circumstances whatsoever. They are an assault on your nervous system and are not at all helpful to comprehension of what you're studying. And that leads to another point: you still need your sleep. Treat your body well.
Good luck.