Whenever possible, make appointments to meet your instructors in their offices. Most universities require their instructors to have mandatory office hours, so this is not at all a strain on their scheduling. It also shows that you're willing to take the initiative to speak in person and solve problems or answer questions most efficiently.
If you can't make a professor's office hours and he doesn't have an office phone to call, send an email. However, remember that email communication should not mean a lapse in manners. Always include a salutation or greeting, text with a short and efficient communication of the problem and a closing. Do not use email to explain any absences, to ask about what you missed or to ask for special favors. Most professors would rather discuss this in person, and email can seem like a cop out.
As stated earlier, non-verbals can have a huge impact on how your message comes across. Take this into consideration and be aware of your body language. Look your instructor in the eye, provide feedback (nodding, smiling) when you understand the message and have the proper posture and attitude. For example, slouching, staring at the clock and not acknowledging the instructor's communication will indicate that you're uninterested and ungrateful for her time.