1. Always remain positive during the interview even if things aren't going as well as you'd hoped. In school, did you ever write a test that you were sure you'd failed, only to find out you passed? You never know, you might be doing better in the interview than you think and you don't want to give up.
2. Try to leave the interviewer with at least one thing about you that might be unique from other candidates that would be valuable to the company if they hired you. Once they've interviewed several people with similar backgrounds, they will tend to look for reasons to hire one person over the others or they might try to eliminate candidates who don't meet certain criteria.
3. If during an interview you realize that the position is not of interest to you, complete the interview and answer the questions as you would if you were interested in the job. I've seen situations where the candidate wasn't good for the job they were interviewing for but the hiring manager referred them to another hiring manager in the company for a different position that they ended up receiving. It doesn't happen often, but it can happen.
4. Don't speak negatively about your former or current employer or divulge confidential information that you shouldn't. I have seen people lose out on jobs by criticizing former employers or by mentioning things about their current employer that they shouldn't have.
5. Don't forgot to listen during the interview! Sure, you are there to answer questions but don't forget to listen. Listen to the questions you are asked, listen to the answers to the questions you ask and also listen to comments that the interviewer makes that might help to shed more light on the job, the company, and your interest in both.