It might seem like adding a layer in the summer heat is the last thing you want to do, but cotton undershirts will actually help cool you down while keeping sweat from saturating your outer layer. Definitely go cotton, since the fabric will wick moisture away from your body and retain it, meaning sweat won't be transferred to your outer layer. Having an underlayer will also create a pocket of air between itself and the outer shirt, which will keep you cooled down when it's blazing outside.
We don't normally endorse V-necks, but for undershirts a grey V-neck will be the least conspicuous option for anyone looking to stay sweatstain-less and stylish during summer's hottest weeks.
Necessary purchases: Gap V-neck T-shirts, $20 for a two-pack