It has been said that a Job Interview is 10% your resume and 90% presenting yourself. Assuming you have the first part down, here are some pointers for the second:
(some of these seem obvious, but I when I did interviews for Circuit City I was convinced otherwise.)

Don't be stiff, don't be sluggish - Remember the three-shake handshake? Use it. Don't turn like a robot, don't wander around.

Sitting vs. Standing - some interviewers might never ask you to sit down. In that case, you stand for the interview, for that is what they expect. When you are asked to sit down, lean forward a bit, make sure your back doesn't touch the back of the chair. Appear as if you are interested in everything the interviewer says.

The interviewer - has been trained to encourage blunders. He or she will listen to every word you say, and if you start making a fool of yourself, encourage it. Do not expect any sort of guidance from them. Maintain eye contact

Dress the part - This varies from place to place, obviously someone doing layout for a web-based business won't dress like a bank's applicant. A tie and starched shirt are prerequisites, jackets make a nice effect (don't put it over the back of the chair.) Shine your shoes, interviewers pay special attention to that. Also, make sure your nails are cut and spotless, and if you are a guy, aren't wearing any jewlery.