Soak your clothes down with ice water. It helps, at least for a while.
Take
frequent breaks... go inside and cool off. Heat stroke sneaks up on you quick. One minute you're thinking, "geez it's hot", the next minute you're going "what's my name?!?"
That happened to me once while off-loading nitric acid into drums. It was hot and humid as, well, Texas, and due to the nature of the acid I had to wear a full chemical suit, facemask and respirator. I suddenly realized that I couldn't read the numbers on the scale anymore and had basically no idea what I was doing. Not cool when you have an open hose in your hands, connected to a tanker truck full of deadly corrosive liquid.
Luckily I had just a thread of common sense left, closed the valve and went and passed out in front of the fan for a few minutes. Coworkers helped me get the jacket and mask off and got me cooled down. I had a splitting headache and didn't feel right the whole rest of the day.
Boy, I sure miss that job...