Practice. Practice, practice, practice.
The most important question is the "Tell me a bit about yourself question" where you give a 5-minute summary of your background and why you're a great fit for the position. You should know every one of the points you'll give in your answer to this question by heart.
Prepare the other talking points that you want to get across during the interview. These should basically be stories of your past successes spun in a way that's related to the field. You can use these stories to answer questions like "What's your experience with <blank>", "Why are you the best candidate?", etc.
Remember, an interview is an opportunity for you to sell yourself-so pick the points that you're going to sell and make sure you know them well.
As for practice, interview with a friend if you can, or a career office if you have access to one. Try not to do it with your partner as that can actually screw you up.
Also practice in front of a mirror. Don't memorize the answers you'll give to questions word for word, but make sure you can deliver those answers well. Practice the answer to the "who are you question" at least a few times each day, and practice the answers to other questions as well.
Look up some really weird/hard questions and practice answering. Your interviewers probably won't ask you anything weird like "What color are you?", but having that sort of practice will give you more confidence at the interview.
If you're into visualization, imagine yourself giving a perfect interview a few times. Imagine the interviewers asking some questions, you giving the perfect answers, the interviewer being really impressed. This will condition you to be in the right frame of mind during the actual interview.