Never, ever leave your dog in the car;
Make sure your dog has unlimited access to fresh water;
Make sure your dog has access to shade when outside;
Take walks during the cooler hours of the day;
When walking, try to stay off of hot surfaces (like asphalt) because it can burn your dog's paws;
If you think it's hot outside, it's even hotter for your pet – make sure your pet has a means of cooling off;
Keep your dog free of external parasites (fleas, ticks) and heartworms – consult your veterinarian about the best product for your pet;
Consider clipping or shaving dogs with long coats, and apply sunscreen to your dog's skin if s/he has a thin coat.