Not that I have been to any job interviews, but we had something about them in school.
First of all, relax. Try to go to bed like 1-2 hours before you usually do. I don't know which job it is, but I am (almost) sure it wouldn't hurt to put on some nice clothes, not just a white tshirt and jeans.
Well, good luck!

EDIT: Just realised it is today (or was), so forget about the sleep thing