One simple thing that students can do to improve their grades, is to read their textbooks and read them at the right times! I am going to give you a 5-step formula that is very simple, but will make a HUGE difference in your understanding of new material and in your grades. This formula should be used every time you are tackling new material for your class:
Step 1: Consult your syllabus the day before each class, and see what is going to be covered in your next lecture. Then, read the corresponding textbook material during the 24 hours before the next class. Take notes using the Cornell method, and leave spaces between each topic, so you can add to them during the lecture, if needed.
Step 2: Go to class, and during the lecture, add to your notes any time you see things you missed, or if you need to clarify things.
Step 3: Review your notes (including the material covered in class) within 20 minutes after the lecture. (If you go to another class right after that one, review while you are waiting for the next class to start.) Edit, or add to your notes, as needed.
Step 4: Conduct another review of this material within 24 hours, and write study questions for the material.
Step 5: Review again in a week and any time you have a chance, go over your study questions. (See this post on portable flash cards.)
Keep track of these reviews in your planner, or each time you finish a review, write the day you should conduct the next review at the top of the page. After you have completed these five steps, you have established pretty solid bank of memories you can draw from during your next exam. If you will be having a comprehensive final exam, continue to skim/review the material every 3-4 weeks to keep it active in your memory throughout the semester.
This process is tight. You will learn and retain information better than ever, and will be far less stressed that you would be if you were cramming for every exam.
Good luck!