Quite honestly, I did almost nothing to prepare for my interviews. I think fairly fast on my feet, and I did not want any answer to sound canned. Since I did not embellish any part of my application, I did not have to read over my personal statement to make sure I knew what I said. Before the interview, I took a cursory glance at any schools website, to make sure that there was nothing there that was imperative for the interview day. Then I just winged (wung? hahaha) it. This was not a callous attitude, but knowing how I do best. I think the key is to just relax and be yourself. Do not lie, and do not be afraid to admit that you are not sure about something. Just a side note. I do not know if this is legend or not, but I love the story I heard from my undergraduate pre med advisor about a girl who went to an interview and told the people that in her spare tiem she had enjoyed playing classical piano for about ten years. They invited her to play during lunch. Needless to say she did not, according to legend, get in.
Just be yourself, know yourself. If you would feel better doing a ton of preparation, then do so. but as I said, I think it is better just to go in and be conversant.