- Know why you want to go to the school - Yes, everyone wants to go to the best school, but why specifically the school you are interviewing with? If you just want to be around smart people, you can go to any of the Ivy Leagues. Know something about the school that is specific to it and would be great for you. Do some reasearch, use the internet. It isn't hard to come prepared.
- Have goals - You don't have to know what you want to do for the rest of your life, but you should have some idea about what you want to do and how attending the college of your choice can help you get there.
- Know why you stand out - Everyone has great grades and test scores. Be able to articulate why you are different from the crowd.
- Talk with passion - people love to here othes talk with passion about the things they love. If you love playing the oboe, show it!
- Demonstrate a strong desire to go to the school - Alumni who interview for their school feel very passionately about their school and have a lot of pride. They do it strictly on a volunteer basis and it is often inconvenient for them. They want to see people who will love their school as much as they do, so show some enthusiasm.
- Have a conversation - don't let it be one sided. Ask lots of questions. People love to hear themselves talk. Asking questions shows interest (which helps with the ones above). When asked a question about something, respond, and think about how you can ask a question right back. This creates good conversation and a natural flow.
Other than that, relax. Like I said, the interview really isn't that important.