If you have a long distance to go just remember that you're losing more than water in sweat. A sports drink before the ride and a diluted one during the ride can really help, especially if it's got some protein in it and not just carbs. A sports drink slushy before the ride is great idea too.

There are also safety vests, shirts, touring shorts, etc. that have pockets suitable for carrying ice packs.

Just being in good shape helps your body deal with excess heat.