Meet with your professors during the posted office hours. The misconception is that most professors (especially those at the graduate level) will turn you down for help, but that is often not the case. If you do not understand a concept or have been performing poorly, you should always ask for help.
Participate more actively in discussions --- not only in subjects with which you are familiar. If you need clarification, ask for it.
Set up a study schedule and adhere to it. Perhaps you are not spending enough time on a certain subject. You may need to reorganize your strategy. Setting up a schedule will also assist you --- particularly to avoid procrastination.
Attend or create a study group. This is not only a great way to get a fresh perspective on a confusing concept but a study group will also introduce you to people with similar interests.
Reduce surrounding distractions when you study. Turn off the phone or television, if you want to avoid interruption. However, some tactile learners may need ambient noise to concentrate. For these students, playing some soft music in the background or having the television on at a low volume may be soothing.
Review your note-taking habits. If you feel that you need to improve your note-taking skills, visit a tutoring center on campus for additional assistance and useful tips.
Do not miss class unless you have an emergency. This is especially true in the case of graduate courses, whose meeting times are often limited due to a student's work schedule. Missing one class could translate as missing a week's worth of lecture and discussion.
Be positive and do not give up. The worst thing is to feel that you cannot do anything to improve, which is not the case at all.