I would buy a cheap plastic kid's swimming pool and have the dog take breaks in it. Make sure the dog's belly and chest get wet, it's also good to try to get betweeen the rear legs wet if you can. Otherwise, just take some good breaks in a cool car or inside, offer nice cool water (I'll stick ice cubes in it on really hot days) and deal as best as you can.
All of this, of course, is after the dog has been cleared of any fatigue-causing illnesses.
All of this, of course, is after the dog has been cleared of any fatigue-causing illnesses.