Get a good nights sleep, eat breakfast, brush your teeth after eating, dress nicely but don't over do it, be confident, have a little 30 second commercial about yourself (nothing outlandish, just straight and to the point), don't BS the interviewer, remember you are trying to sell a product (you) to the interviewer, make sure your product stands out, always smile, have a good firm handshake (not limp, but no he-man stuff either), thank the interviewer at the end, make sure and ask about the hiring process (such as how long until a decision and/or what is the next step), a follow up phone call in the next few days is always a good thing too, even if you get voicemail it shows you're interested.
It really all depends on the type of job you are applying for and what kind of person the company is looking for. Some employers want someone right out of school so they can train them in their own way, without already established bad habits. Some want an employee with a little seasoning who can jump right in and know what to do right out of the gate.
The best advice I can give to anyone getting ready to enter the workplace without knowing any of those details is to think hygene. For whatever reason there are still tons of people out there who have no idea that they even have a problem in this area or what the proper eticate is for office hygene. You want to smell good not just for the interview, but every day while working around others. No one wants to work with anyone else that smells like crapp. Deoderant, fresh breath, and a pleasant cologn or aftershave every single day that is not over powering or pungent. Hair nice and tidy, clothing that represent the image that this company wants to promote to its cliants, no visable tatoos or piercings, and an overall clean cut appearance.
Like stated previously do a little bit of research on the company, and the department you wish to work for so you know what your getting yourself into. This will also help give you confidence in yourself, and in your abilities to fullfill the duties of the position you wish to fill. Being a little bit nervous is OK, but if you look scared to death it will send a message that you are incompitent. You want to look like you are ready to jump right in and help them accomplish whatever it is they are looking to do.