Hi, I know a lot of this information is available elsewhere, but here is what I did.
When I began prepping for interviews, I first started looking at the school's website. Basic info here, but sites are usually up to date, and you can generate a lot of questions of your own for your interviewers. These are important, as they show your interest in their program. Also look at any guidebook or literature about the school if you have some.
I then took the Princeton review book "complete guide to medical schools" and went to the chapter on interviews.
They have about ten pages of common interview questions. I prepped every one of these by writing down an answer in a note book. Not to make a "canned answer", but to get my thoughts together on a wide variety of subjects.
Then I had a mock interview with a couple of my professors. I also went over questions with a friend.
Don't forget to review your application materials (personal statement, EC's and your grade history).
Finally, check out the interview database here as studentdoctor.net, and any others you can find. I found it to be accurate, and it gave me a good idea of what to expect.
Sorry I was so verbose, but this all worked for me. I got my acceptance letter (for Early Decision) last Saturday.
Just relax and be yourself, you'll be fine.