1.) Dress professionally - I don't care if you are interviewing to be a part time secretary or a CEO of a huge company, you are asking someone to become business partners and to give you a paycheck at the end of every week. The least you can do is look professional and wear a suit, if you don't have a suit, wear a shirt and tie, if you don't have a shirt and tie, tell the potential employer you don't own one, but you will buy one with you first paycheck and never make the same mistake again. I just had a potential candidate interview and he came in with sneakers. He actually did ok in the interview, but if he can't look presentable meeting with a potential employer for the first time, he'll give the same lack of respect to our customers.
2.) Be Interested - I mean, you are going to spend 8 hours a day at this place, you should be interested. Make sure you smile and answer the questions enthusiastically, more importantly, ASK questions at the end!!! Ask where you can be with the company in 5 years if you do what you're supposed to do, ask what the toughest part about the job is. BUT WAIT until you get the job offer to ask about money and negotiate!!!
3.) Ask for the position - Ok, this is awkward for most people, including me, but you don't say 'can I have the job?" You go about it in a different way, the whole point of this is to show the company you're interviewing with that you are a serious professional candidate ready to work and be a producer. Close the interview with, "It was really great to meet with you, the job really interests me and if you gave me a shot, I won't let you down."