Dr. Will,
Okay, I know that people are going to say I'm crazy, but I maintained this attitude for every one of my interviews, and each time it worked wonderfully. . .
If you can, try to view the interview as a "fun experience." If you focus on the importance of the interview, in terms of "Oh my God, I've worked so hard, this is what determines everything. . . what if I screw up?!" you'll overanalyze everything that you say and will come across stilted and unconfident. In addition, you'll just have an awful time!
I prepared the night before, as all here have said. . . it's a good idea to read through the information on the school, and to read interviewfeedback.com. Look through your application materials again, find any weak spots, think of questions that you'll probably encounter. Take a look at the news, if you don't regularly, and make sure you know the lastest. One thing that I always did, as well, was to write down at least three questions about the school that I wanted to ask the interviewers. Yes, of course, I wanted DESPERATELY to get accepted ANYWHERE, but when I wrote out questions and decided to view it as, "Okay, this is not just about them analyzing me, it's also my chance to evaluate the school and decide if I would be happy there," a lot of the pressure lifted from my shoulders!
I'd highly recommended what Dra Foxy suggests...when you arrive at the interview site, put on a happy face and chat with those around you! Ask where people are from, what they're studying, etc. It might also help to come early, check out where you have to go, and then LEAVE for a while. .. go sit in a bagel shop w/ a cup of tea or cocoa and chill out, or take a stroll around campus.
If you try to view the day as an opportunity to meet some new people, to learn about a new place, and to have potentially enlightening conversations with doctors, a lot of your stress might dwindle. This comes from my own personal experience, however, so others may vehemently disagree. But personally, once I'd decided not to worry about them, I found interviews exciting! An interview is an interesting way to learn about a new environment, and about new people.
The more calm and relaxed you are during the interview, the more clearly and confidently you will speak. Questions were thrown at me to which I didn't have an immediate answer. . . but because I was comfortable, I didn't have any trouble thinking for a moment and deciding upon a feasible and genuine response. If I'd been sweating bullets, I know that circumstances would have been otherwise!
Best of luck to you, Dr. Will. Just go in there, be yourself. . . and show 'em how to shine!