An interview is THE time to "sell" yourself.
You need to look at interviewing as a potential sale--and you (your skills and experience) are the product.
By being prepared, focused, and confident in your abilities you are much more likely to get the interviewer to "buy" the product--i.e. you will land a second interview or better yet receive an offer for the position.
Preparing for an interview is a lot like studying for a big test--you have to do your homework.
Learn about the company, its mission statement, its history, find out what products they offer, where they're located, discover the management and personnel office--basically anything and everything.