There is a bunch of excellent advice given here.
Some things that I will add is research the company thoroughly. Know the history, know what its SWOT is (Strengths, Weaknesses, Oportunities, Threats). Be yourself, ask questions about the company (bring a list on a notepad) Take notes. basically interact with the interviewer more than just answering questions that they may have.
They may have questions for you that you have no clue on, and just answer them truthfully and tell them you don't know but will research and find out. (and do the research, so when you get a follow up interview you will have the answer) This will impress them. Be prepared to answer questions like "Tell me about a time that you failed at something, and what did it teach you"
Ask them to give you a run down of a typical day on the job (even if you have a good idea of what it entails) Do not ask anything about compensation or money matters. Ask them when they will make a decision and contact you. once that date comes and goes, give them a follow up call to show you are interested in the job still.
When waiting for the interview, talk to the receptionist or person in the office if they are not busy. you might learn a few things from them about the company that you would not know from the internet etc..and the receptionist is usually the first test of any job interview, if they think you are not likeable, not worthy, you could get the ax right there before the interview even happens, but if the receptionist sings your praises, you got an extra plus.
Hope this helps, and good luck!
AJ