Research all the faculty to find out 1) who you're most interested in working with, and 2) to be able to ask good questions. Make contact with those who meet criteria number 1. Be yourself; don't kiss ass, don't dress up preppy if that's not what you normally do, be polite, and mostly just be really interested. If you're not, you shouldn't go there anyway. If you are, you've got a great shot at getting in. If you've made it this far, you've got very good chances.

I've been on admissions committees for the last three years. What I can tell you is that if you have an interview, then you look good on paper. Therefore, they are going to want to learn more from you. Be able to talk about the research you've done. Not just the techniques you did, but also the hypotheses, implications and future directions, but especially hypotheses. Also make sure to ask your interviewer about their research, even if it isn't within your area of experience or interest. You can also ask questions about the school or the program, even if you've heard about it from other people before.

Make sure you are polite to everyone, and get to know some current grad students. Word gets around, especially if someone comes off as abrasive or says something inappropriate.

Try your best not to be nervous. There aren't going to be any trick questions. Good Luck!