If you're 'competitive' (whatever that means), then USAFA will mail you a packet. I imagine if you have certain SATs, GPA, etc., then they mail you a packet.
Everyone summed everything up, so I'll give you my personal story.
I made it clear to my congressman that I wanted to fly and he nominated me for both USAFA and USNA. This is just my .02, but if I were you, then I'd tell my congressman what I want to do in the military (ground-pounder, pilot, lawyer, etc) and let them make their decisions accordingly. Just tell them you went to USNA's SS, but not USAFA's, so you're leaning toward USNA, but you'd gladly go to USAFA.
I think they want people who aspire for a certain MOS, rather than people who don't know what they want from the military or who don't have specific goals after the academy. That was my .00002 cents, but it worked for me.
Also, get started on your letters of recommendation ASAP. And if someone doesn't complet an LOR in a couple days, start looking for an alternate. I found that everyone I asked to do an LOR was done by the end of the day. Someone forgot to write an LOR for me, so I missed one of my congressman's deadlines. I thought 'gee, it takes a lot of time to write an LOR, so I'll ask the minimum amount of people to do it.' Don't do that. Be selfish, get a bunch of LORs and choose the best three.
Good luck,
-Ace