i used to have this problem like 2 months ago. i couldn't wear t-shirts because i'd always have pit stains where ever i'd go because of it (even when it wasn't hot out).
i tried products to dab on your underarms. i tried switching deoderants, ect...
finally, i cured it. i trimmed my arm pit hairs. i didn't shave them off or anything, mind you. i just trimmed them a bit so it wasn't as hairy, and it worked. i'd take a shower, put on my anti-persperent every morning, and now i never sweat. i don't know if it was because i had too much arm pit hair, and the anti-persperent was finally taking affect, or what. but now i only have pit stains after i come from the gym. something to think about, or try, if you're really interested in getting rid of it.