As to the prevention aspect, I will add, use deodorant / antiperspirant at night. Yes, at night. I'm 42 and all these years I have always put it on in the mornings. A couple of years ago I was searching for a solution to the same problem. I sweat easily and my t-shirts would eventually be ruined and I'd have to replace. Then I read somewhere that you should really use antiperspirant at night before you go to bed. Your body temp is likely at it's lowest (meaning less likely to sweat). And overnight gives your pores time to absorb the antiperspirant before sweating. It helps it work even better at preventing sweat, and, when you do, the chemicals have already been absorbed, so when you do sweat, less staining and residue. Since I did this, the underarm staining has virtually been eliminated.