I know that many of you will roll your eyes when you read this, but seriously, the best way to get good grades is to leave your life outside the door when you walk into a classroom. Your car isn't starting again? Forget it. Your mother's on your case? Don't think about it. Your roommate's a jerk? My sympathies (really...I had one of those freshman year). Don't dwell on it.
Just promise yourself that for the 55 or 80 minutes of that class you'll give it all your attention. Focus.
You'll get so much more out of the class. Not only will your grades go up, but so will your self-esteem and your ability to concentrate. And you'll enjoy the class a whole lot more.
Try it. Focus.
P.S. I wish someone had given me this advice in high school chemistry. Instead of resisting the subject with both feet dug in, I might have actually learned something.
