Hesitation to ask questions may reflect an underlying lack of confidence in one's own understanding of the material. If you're still uncomfortable raising your hand in class, don't let your question go unanswered. Ask a friend, ask a TA or go up and ask the professor after class. If your first attempt at asking a question doesn't meet with a satisfactory response, don't be discouraged from asking questions in the future. All the professors we surveyed asked that we encourage you to talk to them about course material.
Office hours are the best time to approach professors with questions.
However, if you can't make office hours, set up an outside appointment. If it is difficult to contact a professor, try writing your questions down and leaving a note in their mailbox, or, even better, try e-mail. Avoid the mad rush to office hours right before an exam by asking questions as they arise. Go to office hours prepared with specific questions or concepts that you want to address. The TA may be more accessible than the professor and may have a different approach to communicating concepts.