If your computer has an iSight camera (either built in or external), this is also a good time to make sure it can see you clearly. Follow these steps:
Unplug your iSight (if appropriate).
Use a soft, very slightly damp cloth (or lens-cleaning paper, available from any camera store) to wipe the front surface, applying only gentle pressure. Warning! Do not use alcohol, ammonia, or any other chemicals; use only water.
Buff with a soft, dry cloth.