1. Turn off MSN and Facebook while studying. You know how sometimes when you are playing a sport or working, you enter "the zone"? The Zone is a zen-like state where you are completely focused on the task at hand, and you are performing it at peak effectiveness. Aside from the fact that multi-tasking is inefficient, if you have MSN open while you are doing your work, you will never be able to get into the zone. And trust me, you WANT to get into the zone.
2. Twist assignment topics to reflect your interests. Say you are a sports fanatic, and you get an assignment on Women throughout the Century in history class. Ask the professor if you can do your report on women athletes throughout the century, or something similar. Make sure you ask your professor first, because this isn't always an option.
3. Set precise academic goals for yourself. If you really want to change your grades, you need to set specific goals. "study harder, get better grades" is not good enough. Set goals like "Get over 80% on the final exam" or "Increase my average by 3% each semester". After you set your academic goals, post them on your fridge, by your bed, and anywhere else that you will see them often.
4. Write down all the things that are helping your marks, and all the things that are hurting them. Focus on increasing the good habits and decreasing the bad. If you can successfully change your habits, your marks will surely follow.