You can buy or make absorbent bra liners — you can even use a folded paper towel. Change it in private as needed. I cannot stand sweaty fabric sticking to me! As a last resort, you can apply antiperspirant to the underside of your breasts. I don’t like doing that, but if you’re going to be outside in extreme heat, like at a festival or something, it can save you from having your poor breasts trapped in sweat-soaked prison. Yuck.