There are lots of threads on this topic through all the Forums, but just a couple brief thoughts (from the guy on the other side of the table).
1) Relax and be yourself -- the interview is no fun for either of us if you are uptight or are trying to hard. (One sign that the interview went well is that you realize that 1 - 1/2 hours has passed and it seems like minutes).
2) Be prepared; Have some good questions of your own,, and be prepared for the most common questions , ie. "why do you want to attend USNA ?"; what would you do for Service Selection ? What do you want to major in ?
3) If USNA isn't your first choice, be honest but prepared to explain why ? (This is addressed in another thread recently and some will disagree ).
4) Don't take everything you see about BGO interviews on this Forum as fact. I don't know the number of BGO's , but suspet there are at least 1000 around the country, a while we have some common training, we all do things slightly differently. (For example, I do most of my interviews at my office....I know a lot of BGO's go to the candidates' home).
5) Finally (this is more personal opinion, as Admissions really doesn't tell us how much weight they give to the BGO interview), except in rare circumstances, the BGO interview isn't going to get you into, or keep you out of, USNA. I have often said here that I suspect the interview has its most effect at the extremes ..ie. identifying the really exceptional or really poor candidate. Fortunately, I've seen alot more of the former than the later.
There will probably be a lot of good (and bad) advice on this thread ...in the event you see conflicting advice, just look at my first comment --if you relax and be yourself, you will do fine !