This may not work so well in humidity but I will stop at parks and soak my jersey in water periodically(wringing out the extra to avoid drips) on 100+ degree days. Keeps me nice and cool until the shirt dries which can be as quickly as 10 minutes if it is a very hot and dry day. I figure it helps me conserve water as my body then doesn't have to sweat as much.

I also find that if I am around very cold air conditioning it goofs up my efforts to adapt to the heat. Try using less air-conditioning if you can, you are probably like me and sit in a very cold office environment all day long.