Legend has some good advice.
Here is some more.
Get a haircut the day before, even if it is just to trim it up. make sure you are clean shaven (or your beard/mustache is neatly trimmed)
Wear matching belt and shoes. if the shoes are black, the belt must be black. (see, us guys get to accessorize too

)
make sure that the shoes are cleaned and shined up. make sure your "gig line" is lined up (the strip of your button down shirt should be in line with your belt buckle and the trouser fly
ok, enough of the fashion police... and no I am not gay
Now the thing you need to do the night before the interview is research research research! get on the companies website if they have one and look at their history, who thier cutomers are. Go in with this knowledge and you will impress them. Make sure you think of some questions to ask them too. When they ask you: "Do you have any questions?" make sure you have some. this will tell the interviewers that you are interested in the company, and it will feel like a two sided interveiw instead of them doing all the questions. Plus you will learn more about the job and know if you want to accept it when they make you an offer.
Go in with several copies of your resume. they will have one, but it is nice to be able to give them an updated version and give copies to the other interviewees that might not have a copy.
Remember when you go in, that is the impression that they are going to form of you. make it a positive one. give good eye contact, be courteous to everyone, including the cleaning lady or receptionist. they might be there as the first part of your interview. how you interact with the "peons" will be noted.
During the interview, take a notebook and take notes. write down the names and positions of each person that interviews you. jot down things that you learn about the position that you might want to revisit in a second interview, or ask about later. be prepared to answer some questions that you have no clue how to handle. be honest, tell them you are not sure how to do that, but you will research it and get back with them. (and do it, they will probably ask it again in a second interview)
After the interview, be sure to thank all interviewees for their time, and send them a "thank you card" in the mail addressed to each interviewee a few days later. This will show your professionalism.
Never ever discuss wages, compensation or other benefits until they make you an offer. This again will show your professionalism.
Good luck,
AJ