It is a fact that the brain is more open to suggestion in the morning. You've just waken up, and you haven't got a lot on your mind. Also, certain stages of sleep resemble hypnosis. So, I study in the morning. I retain things better. All the weeks before a test I will study consistently, and then the morning before I will get up very very early, and read all the chapters and notes over again. That way everything is fresh on my mind. Sometimes I re-copy or type out my notes.
If you are having trouble in a class, make sure that you talk to the professor. If s/he knows that you care, that could definately affect borderline grades. And you may get some extra credit.
One of the biggest things I've notice in my lengthy college career (all of one semester

) is that it is really important to go to class. Most of the grades that I made had nothing to do with work, but with attendance. I made one 'B' last semester and that was in Sociology. I hated that class and NEVER went. Everything else was made easy because I went to class. ALL of the people that I know have failing grades skip their classes.
Get a planner. Schedule your time well and you can get tons of studying in. If you are giving a presentation or speech, try to go first because the teacher (usually) is more lenient in grading because you didn't have as much time to prepare. And you will feel better because you have gotten it over with. And most important -
Don't procrastinate!!!
Hope all this helps! Good luck!
Allie