Sarah Hall said:
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.
And you want ONLY the parts of the dog where the blood vessels are exposed to the water (so to speak) to get wet. So the face, groin, belly, etc., but NOT the furry sides and back.
I learned this from Bob Scott and others (about soaked fur holding heat in and making it worse), and now Leo's and Pomfret's clubs have pools with water levels appropriate for the size of the dogs. The little dogs' pool has only enough water to soak them paw-to-belly, and ditto the higher big-dog pool.
Huge step we took at the club: reflecting awnings for the shaded part. I couldn't believe the difference between the old green canvas ones and the new reflecting ones.