Make sure that they always have fresh, cool and clean water to drink, so that they stay hydrated.
Dogs should be in the shade where it's a little cooler, and out of the sun. If it gets really bad--really hot out--the dog(s) could be brought inside the house where it is safer. Dogs are members of the family, so it might be a good idea to bring them into the house, so that they don't have to be outdoors in the unbearable heat.
When it's really hot dogs shouldn't walk on the pavement, or any surface that is hot, because it might burn their feet.
Dogs need to be on "Heartworm" medication to prevent heartworm disease.