Definitely take advantage of the SDN interview database -- it's the best I've seen. It gives you sample questions that will help you prepare as well as a summary of what to expect at the interview.
Then practice, practice, practice. The more times you go through the interview beforehand, the better you'll do on the actual day. Try to tape yourself if possible -- you'll get a better sense of whether you're rushing, saying "uh" a lot, or any other detracting things -- as well as practicing with others.
Good luck!