E-mail is a great way to make an appointment. If you want to see a professor and not during official office hours, drop in on him or her anyway. If not otherwise occupied, he or she will almost always be delighted to help you. Study with others, either with your friends or as part of a study group. One of the best ways to learn material is to teach it to others.
Study groups are a great way to study for exams or work on problem sets and lab write-ups.
They range from regularly scheduled study groups arranged by a professor or the Quantitative Skills Center to a group of friends who decided in class to meet later that night. Everyone in the study group should work on his or her own before going to the study group. You should have already attempted all of the problems or studied on your own for the exam. A good way to help others in the study group is to ask questions that lead them to the answer. It's a much more active form of learning for them, and you can help without giving the whole answer away.