+1. Any good interviewer is going to be looking at the details of your appearance more than what you actually wear (unless you choose not to wear a suit, which is bad). Nails/nosehair clipped? Shoes don't look like they're 5 year old hand-me-downs? Suit pressed? Etc.
After that, it's how you answer/deal with questions rather than what your responses actually are (unless you say something exceptionally f'd up, which is bad).
Your appearance won't win over the interviewee, but it can easily lose it for you before you open your mouth.
There is great advice on here, though a Gucci suit will look the same as a no name rip off suit to them, as long as it is approproiate for your frame. Though a t-shirt and shorts will say a lot about your attitude towards the program. Spekkio said it best, especially with the finger nails and nose hair, don't forget about those little details. And I would make sure that you do not wear any cologne or axe body spray or even strong smelling soap.
Treat this like you are going for a job interview. Show up 10 - 15 minutes early, anything more is too early, and I have found that bringing a book with you to read before gives you some points before the interview. It usually is a good conversation starter, though I wouldn't bring a military book, but something that is interesting and conversation worthy. I bring a book called the History of the CIA... for every interview, make sure you know what it is about though. Also make sure that you have some good answers to usual interview questions:
What is your greatest strength?
What is your biggest weakness?
Why did you chose your particular major?
Why do you want to join the Marines?
Why do you want to be an Aviator?
Describe an example of an event/instance when/where you showed leadership?
I also always get asked if I am in a relationship. And I have found that you better have a good reason for your answer whether it is Yes or No.
Good luck, have confidence, this should be something fun and exciting, practice with your friends or a teacher, you will do just fine.