Practice, weights, practice, stretching/exercises and more practice.
I played ball from the time I could walk until I was 19 and I still play slow-pitch every so often. It's one of those things where you do it everyday for months at a time, your arm gets built up and used to it.....I could throw 100-130 pitches in a game in HS/American Legion and only have a little soreness the next day, due to lactic acid build-up, not actual throwing.
Get some surgical tubing/resistance bands and use them to do stretches/exercises to work the arm/shoulder. Throw catch at varying distances as often as you can (every day), etc. Do NOT, and this is non-negotiable, do NOT try to throw hard/long distance on a non-warmed up arm.....fastest way to damage it.