I third the cold wet towels. Any time I have a team playing on a hot day, I use them. Go to the dollar store and buy the packs of cheap utility cloths, slightly bigger than a typical washcloth and similar material. Roll them up, stick them in a small ice chest and throw a little bit of water and a lot of ice over them. It makes a huge difference. Bring them home, throw them in the washer and ready to go for next day. Of course, pitchers and catchers are the biggest beneficiaries but works for everyone.