Tshirt pit stains - how do I get rid of them?!?

Whenever I wear a white tshirt, after 1 or 2 wears, a yucky yellow stain appears under my pits - I wash the shirt and the stain just stays the same and then I wear the shirt again and the stain gets worse... anything I can do to get rid of the stains? I already know of my sweating problem and already use Dry Sol and have already been to the Dr re: botox so I don't want a lot of answers as to WHY I'm getting the stains - I just need help REMOVING the stains :D Thanks for your help! I'm tired of throwing away tshirts after 2 wears!