Well, living in Florida I know what the heat is!
To keep Beenie cool (she has short hair but it's black - heats up in no time), I wet her head every so often when we are out and make sure she has plenty of cool water available. We just got back from a short walk and she seemed to be a little overheated. I just sprayed her with water (from a spray bottle) a little and she seemed to feel better right away. I don't know what I would do If I had a long haired dog. I'd probably shave it's belly as was said earlier in this thread.