Got this from the Yahoo! Pets site:
Doggie Boots: Dogs absorb and release heat through their feet. Boots protect your dog from the powerful heat that rises from surfaces like cement and asphalt.
A Kiddie Pool: Got a nice spot in the shade where you can set one up? It's a fun way to let your dog cool off. (Just like my pal Baylee, shown in photo!)
A Wet Towel: An inexpensive way to give your dog some relief from the heat. Remember, dogs cool from the bottom up. A wet towel does more good on the bottom of your dog than on top of her coat.
A Water Spritzer: Spraying your dog with cool water is a great way to cool your dog down on a long walk. Spray the paws and stomach too -- not just the top of the dog!
A Portable Water Bowl: Always have water available for your dog on long walks. Some paths may have a special water fountain for your pup, and that's great. But your pup may need water before you reach it. A portable water bowl and a bottle of water is a great way to ensure that your pup won't go thirsty.
A Bottle of Water: Be responsible: keep one handy on a long walk. If your dog wears a backpack or vest, you can even let him carry it. The water inside the bottles will keep the dog cooler, and carrying it will give him a sense of purpose.
Remember, we can't speak up, so always pay close attention to our body language to make sure we are not over heating!