Attend classes. While this seems like an obvious step, skipping class is not conducive to your education. You miss out on valuable information not covered in the textbook. Some professors also include attendance as a part of your grade, especially in smaller classes.
Get a study buddy. Ask one of your friends from class to compare notes. They may have picked up on something you didn't during class. You can also assign sections of reading to split the work load, especially when carrying a heavy course load.
Decrease your responsibilities in clubs and other activities. If you're an officer in a club or captain of a sports team, step down to dedicate more time to studying. If you have a job, consider cutting down on shifts to give yourself more time for school.
Ask your professor for study guides or extra credit. He might assign you an extra paper or project that makes the difference between passing and failing or whether you make the cutoff to stay on your sports team or continue to receive your scholarship.
Drop a class. If you're taking a class that is too difficult or your grades are too low to bring up, drop it for the semester and take it another semester when you can dedicate more time to it or with a different professor whose teaching style you understand more.