Each senator/congressman has their own system of determining who they are going to nominate, which could be totally different from another MOC (member of congress). I suggest you follow each senator/congressman's instructions as closely as possible.
Everything you do during the application and nomination process will reflect on you. If you deviate from the instructions given, to make yourself look like a better candidate, it could be good or bad for you. It could show that you took initiative, or conversely show that you can't follow directions.
Jonathan: You asked where to send your letter so you will have more of a chance of getting a nomination. The only place to send your nomination application is to your 2 senators, your representative and the Vice President. What happens after that depends on which academy you apply to, how many openings the MOC has to fill, how many other applications each MOC gets for each academy, your test scores, GPA, ECs, recommendations (if required), etc.
It's a tough process.
Good Luck