Check out the Armstrong program for pullups. It works. I was stuck at 16 pull ups no matter how hard i trained until I started this program. http://manlypat.wordpress.com/2009/02/24/armstrong-pull-up-program/

I will also repeat what Randy said (just don't tell him I agreed with a chairmen), RUN RUN and RUN SOME MORE. This isn't that important if you plan to join the Air Force or Navy, but it is still important. If you plan to join the Marines or the Army, it is very important and will affect your career progression as will your situps and pullups (Marines) and pushups (Army).

Do not just focus on those three or four exercises though. You need all around strength to perform well in the Army and the Marines. The Army is going to adopt our Combat Fitness Test, so all around strength is needed. This includes Movement to Fire - timed 880 yard sprint in boots and utes, Movement Under Fire - seen in the video below and ammo can lifts - 30 pound can lifted as many times as you can from below the chin to straight up arms locked out in 2 minutes.




http://www.youtube.com/watch?v=cuw7fh4zfck