I did an interview for a slot at the Naval and Air Force Academy, which I imagine is pretty much the same thing as the ROTC interview. (That was 14 years ago, and I'm a Major now, so you can guess that it went pretty well!)
In addition to what was already posted above, I would add:
1. Make it clear that you are doing this because you want to serve your country, (not because you want money for college, or because you want to be a pilot, etc., etc.,). The patriotism angle is probably something key that the interviewer is looking for. You can say, "in addition, the money for college is a great added benefit, and I'm very excited about flying if I can," but you want to lead off with, "I am applying because I want to be an officer in the military. I am proud of, and want to serve, my country."
2. Don't get thrown by unexpected questions. I was ready for the questions about what I wanted to do, my record in school, my extra-curriculars, SAT scores, etc. The question that caught me unprepared was, "what would you do as an officer, if a superior gave you an order that you didn't agree with, but seemed to be generally legal? Suppose instead it was an order to kill a prisoner?"
3. Try to be/act/look/speak professionally and politely. Erase, "Dude", "Man", "Cool" and words like that from your vocabulary until the end of the interview. The military look would probably help, but you don't need to go overboard with it. In other words, you don't need a crew cut... but you probably don't want a pony-tail (as a guy) either! They are looking for clean-cut, well rounded, smart, Joe-american (or Jane-american).
4. You don't need to be totally "HUAH", and in fact that would probably be taken strangely. Aim for positive, interested, and enthousiastic, without being too over-excited. Guys who are IN the Army or Marine Corps sound retarded when they answer questions with grunts and "Huah!!!!!". High School or College kids who aren't even in YET will sound even stranger... and come off as slightly unstable. (Of course, I always thought that the Army and Marines were a bit unstable to begin with...
).
5. Do some research to show that you've looked into it, but avoid sounding like you know what the military is all about or being a "know-it-all". 99% of what you think you know about the military lifestyle is probably wrong anyways. Trust me on that one. The interviewer will more than likely be able to tell the difference between the correct and the incorrect, but it doesn't matter to you as the interviewee, anyway. They don't expect you to know every in and out to the military lifestyle anyway. They want to see that you've done some research into it, but are aware that it is probably not going to be what you expect, and you are eager about it, even though you don't know 100% of what to expect.
6. You know how a lot of geniuses are total wackos? Like how Van Gogh cut his own ear off? So does the interviewer. Eccentric genius is less desirable than generally competent, smart, above average, but otherwise psychologically normal and stable citizen. This sort of ties in with #4. They want above average, well rounded people, not people who are so far above average in one area that it's made them "strange" in all the others.