Have a talk with your professor. Try to get a sense of what he or she thinks is important and find out what he or she would like to see. It cannot harm you, and it may do you a world of good. By discussing your work and explaining what you were trying to get across, you give the professor a chance to see how hard you've worked. And professors have at times been known to change the occasional grade if it is based on subjective material and you can convince your professor to re-evaluate your work.
Most schools offer free academic help to their students.
Maybe you just need some tips on how to improve your study skills to make better use of the time you currently put into your classes. So make use of your school's study skills counselors - they will be able to help you identify and work on your weak areas. If you've tried everything else and you are still unsatisfied with your grades, take a look at your schedule. For starters, review your course load. If you are taking too many credits in one semester or too many difficult classes at the same time, your grades are likely to suffer because you won't have as much time to devote to each class.