Dress code: Business casual, just a neat shirt/T-shirt and pants will be just fine. Don't wear clothes that have extreme paintings/colorings; just plain and simple. Shoes need to be clean and comfortable, don't wear new shoes. That doesn't mean you have to wear shoes with holes showing your toes either.

Homework: Take a look at research the School does. If there's something interesting going on, learn more about the faculty and what he/she does. It's nice to have something to talk to the interviewer's about their institution. If you know who's going to interview you, learn about the type of research he/she does. Be prepared to explain your prior research experience in a concise yet effective manner; practice if you feel like you'll lack fluency during the interview.

During Interview: Act professional, give honest answers but don't feel obliged to reveal personal information, listen to the whole question before jumping in to provide answer. Comment on the interviewer's research and/or the program you are applying if/when you get a chance - very important! Have fun.

Source: based on my talk with a graduate school admissions administrator at State's one of the best research schools.

Good luck!