Show up on time, bring a resume. Don't worry about dressing up; I interviewed in a tshirt and shorts. Just like with any other job, good grades and impressive involvement/activities will help you win them over. Emphasize any customer service jobs in the past. Also, they might ask you where you get your understanding of cardio and lifting and exercise in general; it'll help if you have an identifiable source (anything from Men's Health magazine to an exercise plan from a personal trainer). After a couple weeks I got an email offering me the position. TAKE THE JOB. The pay is literally minimal, but you'll meet great people, learn a lot about fitness, and become a recognizable face on campus.

The interview was an open, walk-in interview. You fill out a piece of paper and the one of the management personnel interviews you 1-on-1. They ask about an previous experiences, why you want to work there, etc. They give several short case scenarios and then interview is over.