...I suppose you want a "real" answer and not Tippy2K2's smart ass lipping off
1. Look the interviewer in the eye. This is very important as it exudes confidence and let's them know that you are taking this seriously
2. Be confident. See #1. Confidence is almost as important as being qualified for the job. There is going to be competition for your job and you walking and talking like the job is yours is step #1 for it to actually become yours.
3. Have a question for them at the end (and not a "How much does this pay" kind of thing). Have a question that demonstrates that you are interested in this job and the company. You should always have a question for them for I have never seen an interview where they don't expect one.
4. Scarim Coral already said it but it's important enough to say again: Research common questions. Most interviewers will cover the few base-line questions of "Why do you want to work here", "Name a strength of yours", "Where do you see yourself in X years" (Don't say doin his wife...) and "Name a weakness of yours".
5. Think of examples in your own life of handling a tricky situation because that question is almost guaranteed to come up.
I hope some of this helps. Now I'm off to my job!