In the spirit of the interview discussions going around recently, and because I'm involved in the hiring process at my company, I figured I'd jot down a quick list of things not to do during an interview.

1. Talking bad about former employers/coworkers- While it's fine when we ask you why you are leaving to say that the environment wasn't to your liking etc, do not start listing off all your personal problems with various people in your former company.  This always reminds me of someone who has been divorced 5 times and it's always "the other person's fault".  You can also give the feeling that you can't deal with conflict.  If you feel you must talk specifically about behavior and traits that upset you, also talk about the mitigating steps you took and would take now if faced with a similar situation.
2. Arrogance- There's a fine line between confidence and arrogance.  Sometimes during interviews an interviewer will challenge your position on something just to see how you react.  If you're hostile, get flustered, etc it can score some pretty big negative points.  Show that you can have a mature dialogue and agree to disagree, since you'll never work a job where you get everything your way all the time.
3. Eye Contact / Confidence- Many people come into interviews very very nervous, to the point where they can't express themselves.  This is something that you need to practice and get over because it interferes with your ability to impress the interviewer.  Remember to make eye contact and speak clearly.  If you have nervous problems then try to practice by getting out in public and talking to strangers.
4. Be clear about the salary and benefits level you expect- Negotiation is somewhat of an art form, but a big no-no is to say you're happy with X range and then when the employer offers X come back and counter with a much higher number.  Lowballing your requirements to get into the interview then hoping to impress and ask for more money usually backfires since you were dishonest in the start of the negotiation process.
5. Don't be afraid to say "I don't know"- The .NET framework is huge, no one knows it all.  You have areas of strengths and weaknesses, don't be afraid to tell the tech interviewer that you don't know, but it's a good idea to tell them how you would go about finding the answer on your own.
6. Show some personality- You will end up working with/for the people in your interview.  Don't be afraid to let some of your personality out (within reason).  We're hiring a human being, not a robot.  Try to take the lead from your interviewer, if they are jovial and humourous, don't be afraid to throw some tasteful humor back at them.