What most everyone has said above me. I will add though that while you don't need to sound professional, you should speak formally, avoid slang and vulgar words if you can.
Be prepared to tell a story if they ask you to answer about "if you had a time when you..." but keep it short and to the point, because it makes you seem a lot more creative (employers tend to like this)
go in with a smile, be happy!
Shake people's hands when you introduce yourself. It's a sign of respect, and trust.
Preferably a full-length tie, not a bow-tie. (they look snazzy, but are almost comical in nature)
If you can, get to know your employer ahead of time, perhaps visit the area once or twice, ask some questions.
Bring a pen in case they want you to fill out paperwork
Your interview will probably be the most formal outfit you will have to wear, if you have a uniform, great! Otherwise, if the employer asks if you have any questions, ask about employee policy, and specifically dress code.
If you can have a flexible work schedule and work when they need you to, make sure to mention this in the interview. An employer likes someone who can work more often.
Generally have a good time and while being formal, and acting formal, feel casual about it as if its second nature
Also, posture can say a lot about a person. Stiff upper lip (or a smile), chin up, shoulders relaxed, try not to stomp around, and keep your back straight (If you can't force it, don't. You should have a natural curve even when your back is straight.)