1. Make sure you have a good idea of the position you are interviewing for and the skills required. It is important so you can brush up on anything not fresh in your mind. Be prepared for technology specific questions. Sounds obvious but don't read "Programming Interviews Exposed" to prepare for a web developer or DBA interview...
3. Never assume you know what a job is like by the HR description (they usual suck). I just went on an interview thinking it was a position I wasn't too excited about. Only to find out while I am interviewing that this is exactly what I am looking for! I made an assumption and because of that I did not prepare properly and now I probably missed a good opportunity.
2. Rehearse the "What have you been doing at your company?" or "Tell me about your last project" type questions. Depending on how many people you meet with during the interview this question will be asked every time...!!! Rehearse the monotonous questions so you are enthusiastic when the 5th person asks you the same thing.
4. If the question is not clear ask the person to elaborate. I hate when I realize after an interview what the person was asking. Its a little late then :-)
5. Don't work before an interview! Your mind will be all over the place.
6. Don't wear a suit! They really do make you uncomfortable and you don't perform well. Suits are gone for me. Sharp business casual is the way to go!
7. Make sure you drink water. You are going to be doing all of the talking.
I know these are "interviewing 101" tips but no matter how much you prepare every interview is different and you need to be able to adapt on the fly.