What would be the best way to improve my IST scores?

Well not knowing what you do already to physical fitness, I would suggest being at the recruiting station as much as possible for PT. If you're in high school, do sports, utilize the weight room. I am a strength and conditioning coach for my local HS teams. I also will help out anybody who wants to join any branch achieve their goals. Also youtube anything fitness related that you want to get better at. There is plenty of information out there. I think also the best thing to help improve any physical fitness is to surround yourself with like minded individuals who will motivate you and push you to the next level and also you to the same for them.

Benjamin Alleman: Mr. Alleman, many prayers, on your success at finding out what you need to know.