Keep this in mind as you go in. You're just as good as any other candidate on paper. Tomorrow's another day. During your interview make some mental notes about the other candidates in the waiting room. If they do pass you over for one of these other candidates, the mental notes you've made about their appearances, demeanor, accessories etc. will help you to be better prepared for the next interview.
Appearance is very important.
Shower, shave, best suit and tie, hair well combed, smile on - look like a competent, relaxed professional. One trick to use is a mental game - rather than trying to make yourself feel comfortable, try and make the interviewer feel comfortable. Besides appearance, confidence is importance. Look like you belong there. Make the interviewer take a second look at you as if he's seen you around the office before.