Be early but no more than 5 minutes early. Really, that's important. Be ready with questions for them and not about time off or pay. Do some research on the company and ask questions about them, the job you'll be doing, etc. Interviewers love it when you know about their company and showing you put in the effort to learn will impress them. Let your personality show, they don't want automatons. Treat the interviewer(s) like people, chat them up. Don't fidget, take notes, ask questions and hide your nervousness.
Like others have said - be confident yet not arrogant. Tomorrow you are a salesman and you are the product - sell yourself to them, let them know what an enthusiastic employee you'll be.
Good luck!!