HopperHome.com offers the following tips in Buns In The Sun:
"Made for Shade: Keep your rabbit out of the sun and have a cage in the shade. Indoor rabbits with direct sun into their cage or pen in the summer need to be protected, too. Heat passing through a window doesn't escape back out so the room heats up. If you let your rabbit run in the garden in the summer, have a shady place for it to rest.
Fans: A fan that will pass a breeze by the rabbit is great and particularly a circulating fan so the breeze isn’t constant. Groucho spends his summer by his own personal fan. Don't put the rabbit's cage in front of an air conditioner and so they really "chill out!" Be sure to bunny proof the cords!
Less Fur: On long-haired rabbits – give them a "hare" cut for the summer. Also, brush hair out of a bunny’s coat so there isn’t extra. After all, that is a fur coat your rabbit is wearing in the summer.
Vegging Out: Vegetables help keep rabbits hydrated so make sure they are getting plenty. Rinse the vegetables and leave the water on them. Be sure they don't get too warm and remove if they look wilted.