You can prevent arm pit stains by wearing darker colors for shirts. Also, a good anti-perspirent/deodorant will help with moisture buildup. Keeping your body temperature down and avoiding hot environments will help you as well.
Speed Stick Deodorant has just come out with a new stick deodorant called "stainguard" specifically for the purpose of preventing yellow arm pit stains. I would suggest you go pick some up at a local store and try it out.
