Wear a tie at the very least and look decent. Buy a suit at Goodwill if you can't afford better. Seriously...you get one shot to make a good impression.
If they've given you some papers beforehand to look at, tell them what you could do to make it better. And if they haven't given you papers beforehand, at least get the latest copy and study it stem to stern and critique it. Learn the area beforehand as well.
Act confident. Look me in the eye and show me you can get the job done. Sell yourself and your work and don't be embarrassed about it.
Never ever go negative on a previous boss or employer -- it will make me think you will do the same to me.
Don't ask how much the job pays unless it's offered up first in conversation, which, if they're up-front, they will (I always said how much it paid so I could weed out the uninterested from then on).
Don't make demands in the interview -- if they want you, you can ask that later.
Whatever you do, don't do what one candidate did to me once -- he interviewed for a prep job, and this guy was so overconfident that he'd get the job (because he worked in the same chain) he said he'd have to fit his work hours around his golf schedule. And believe it or not, he was shocked that he didn't get the job.
Never, ever call the guy who interviewed you for the job late at night at home and wake him and ask him how the job search is going. I had a guy call me once at 11 p.m. at home -- when I was at work -- and he woke up my wife. He was promptly eliminated.
Lastly, follow up with a thank-you note, separate ones to every person you interview with. Even if it's an e-mail one, it's better than nothing. Managers almost never get them and they remember the candidates who do them.
Good luck with the job interview. Hope these helped.