I was in your position a few weeks ago. I have to say that preparing for my interview was one of the most stressful and nerve-wracking times of my life. But you can prepare for it to make it easier. Between the advice given to me by my family, my ALO, and SAF, I was much more prepared than I would have been simply winging it. Most of this post is just paraphrasing everything those sources taught me.
First, make sure you have a general idea of what to say to the common interview questions: e.g. tell me about yourself, which academy do you wish to attend, what's your planned major and what career field are you interested in after you graduate...most applicants are asked these types of questions.
I happened to be staying with my aunt during the weekend of my interview (I had to drive 400 miles to interview). She interviews people as part of her job at a local hospital. The best piece of advice that she gave me was, "if you feel like you're going to freeze up, then tell them a story." I definitely used this more than once during the interview. Interviewers in general like to have background knowledge of you. Give them something good that they can't find in your file.
My ALO (who regularly sits on these nomination boards) could not stress enough that I should try not to flat out say "I don't want to enter career field A because I want to do career field B and career field B only" when they ask you about your planned major and post-grad career field. In his words, you should always appear open to any and every opportunity at the academies. I used this in my interview, as well. I was asked what I wanted to do after I graduated from an academy. I told them if I attend USNA or USAFA that I want to be a pilot, and if I attend USMA I want to enter the intelligence field. There was an Air Force captain on the board who then asked me "So, you wouldn't want to enter the intelligence field at USAFA or USNA?" I replied with something along the lines of "It's not that I wouldn't want to do it. I would certainly be open to it."
In short, be open to everything that the academy has to offer.
The most important thing that SAF and the interview, itself, have taught me is to
relax! I was really nervous when I entered that room, but the interview board understands, and they certainly made me feel very welcome.
On top of everything I just wrote, I have read that other people have been asked military knowledge questions during their interviews. I was not asked any of those kinds of questions. My interview happened to be on the 72nd anniversary of the attack on Pearl Harbor, and I wasn't even asked about that. I would suggest that you do try to know a little bit of military history, though. It honestly can't hurt - and it didn't hurt, seeing as I didn't need to use that knowledge that day.
I wish you the best of luck!
EDIT: The toughest question that I was asked was "Now, take as much time as you need to think about this, but if you had to choose one word to describe yourself, what would it be and why?"