Provide shade, whether you put a blanket over the fence to block the sun, have a dog or pet house for your pets to go into, or you only put your pets outdoors during cooler, shadier parts of the day (early AM or later afternoon).
Always have fresh water available for your pets out of the sun. Put ice cubes in their water to keep it cool all day.
If you have caged outdoor pets (like rabbits or chickens), mist them with a spray bottle or a mister above their cage during the day to keep them from overheating. A fan that blows cool air and ventilates the area is preferable as well.
Bring your pets indoors as the temperatures climb. If possible.
Shave cats/dogs in the summer if they are primarily outdoors, but provide shade as well to prevent sunburn.
Avoid exercising pets during hot parts of the day, saving exercise for the early morning or late afternoon/evenings.