Originally Posted by Nightshade

Brainy don't mean shit you go in as a grunt and you prove you are able to follow orders and make critical decisions under pressure when it counts. You will gain rank as you go and some gain faster than others.

If you brown nose you will be a private your whole life, but if you do your job and report as directed and excel in certain areas the right people will take note.

Also take any and all training that is offered even if you think you will never need it, the more jobs you can do the more valuable you are to your commander and therefore more likely to rank up faster.