S.F.
Dress appropriately, which means clean cut and conservative. Avoid overdone makeup, overdone jewelry and overdone cleavage.
Smile and look people in the eye. Shake hands firmly, but not a death grip. As I tell my Economics students...shake hands like a guy (if you're a girl).
Do your research on the business. Know what they're about and what their needs are. Emphasize how you can help the company meet their needs in a cost-effective, efficient manner.
Don't babble. Take a moment to think through your answer, state your answer and move on.
If asked if you have any questions, make sure your first questions ARE NOT "what will my wages be?" and "what sort of benefits package do you have?" Yes, we all work for money and we all love benefits, but these can be job killers. You might as well walk in with a sign painted in bright, pylon orange that reads, "I'm not interested in your company but only in myself." While it may be true, you should at least hide it until you get the job. I mean, really, would you hire a person with that attitude? Probably not. I worked HR for a company that REFUSED to hire people who asked these questions in an interview. (We made sure to bring it up, though, so there were no questions on the point.)
Your questions should focus on the company first. Then, and only then, should you bring up wages and benefits.
Last but not least, remember that an interview goes both ways. If you interview and decide that this job or job environment is not for you, you may decline any offer. Too often we settle for a job, rather than aspire to one.
Good luck!