Think of your graduate school interview as a professional job interview. All of the same rules apply.
Dress professionally
Bring copies of your resume
Remember the purposes of the interview: to communicate your interest, motivation, and professionalism, and to gather the information you need to determine if this is the graduate program for you
In meetings with current graduate students, try to ask questions to learn what they really think about their advisors and the program. Most students will be forthcoming, especially in one-on-one conversations.
Don't underestimate the potential influence of current graduate students on the selection committee. Present your best side to everyone because every person you encounter during your interview process might be in a position to help or hurt your application.
Ask intelligent and informed questions
Do not ask questions whose answers could have been obtained from a Web site or brochure