I recently picked up a cooling vest at Cycle Gear for $9.95. It works great under a mesh jacket. Soak it in water and it will keep you cool for much longer than a wet T- shirt. I rode for an hour in 90+ temps/100+ heat index. It was still pretty wet after an hour. I'll be trying it on a longer ride this weekend.
If it drys out, just stop and soak it again.
I ride, Therefore I Am.