Know the routine of the interview. We even suggest a role playing exercise with the friend as the interviewer. In any case, practice interview before you go. Write down the time and place of your appointment in addition to the name of the person interviewing you. Wear professional attire for the position.
First impressions are important.
Be yourself, for two good reasons. And second, because few students can play a role well enough to fool an interviewer anyway. Be on time. Be friendly and courteous, but not to casual. Have your thoughts and questions well organized.