Be interested in the school, don't ask cookie cutter questions, and make sure you cover the topics about yourself that you want, so that when the interviewer leaves and writes you up you'll look good.
For example, if you both have something in common that you are really interested in, don't talk about it exclusively even if the conversation is going very well, because when the interviewer sends his writeups to the admissions council (that he/she might not be part of) it'll just say "John is a good kid, and likes to ski."
Look at the application you sent them and select several of what you thing are your strongest attributes (this doesn't have to come from the app, but if you did a good job with that they'll all be there) and make sure that you find a way to fit most if not all into your interview so that the interviewer has something to write about in his report.