Take the ACT as many times as possible. Study for it each time to the extent that you have opportunity. Run for school offices, particularly President positions. Get on varsity sports teams, particularly team sports as opposed to individual sports, and work to earn team captain status. Educate yourself on the CFA, and then work hard to get a great score. Practice the CFA frequently by doing it all within the time frame.
Those are the big things that make up the most significant admissions assessment. They literally score the things at the academies, and while there are other things they may look at, these big ones carry the day. Class standing as opposed to GPA, combined with your superscored ACT/SAT, is biggest. But leadership factor also huge, and a good CFA can mean the difference between you getting in over someone else with a similar résumé.
....also, stay the heck out of trouble.
It seems like most of the kids who get in to the academies have a vision quest.