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:
1.
Unplug your iSight (if appropriate).
2.
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.