A web cam interview?! You mean via Skype or something?
Never heard of an interview done that way.
Off the top of my head I'd consider -
Make sure your computer and connection are reliable, microphone and speakers are set to a suitable volume.
Make sure you are sat properly, with a neutral background, in a quiet place. Ensure plenty of light so you can be seen.
Eye contact would be harder, but ensure you don't look at things around the room etc.
And with any interview, ensure you know about the company and how your skills relate to the job description.
