With it raining all night and this morning, I am *very* thankful for the weather break. Last week we were in the triple digits and it was just insane. I have always had good luck when dealing with heat, but what about those who aren't so fortunate to have a nice barn?
First thing you can do is invest in a fan just for the rabbitry. It doesn't have to be a fancy one, just somethign to get the air moving. Where we live in the Rogue Valley, the air does not move unless you move it! Circulating the air will take out a few extra degrees.
Cool water is extremely important, wether you feed from crocks, bottles or auto water. A rabbit will not drink warm water.
Ice bottles. No matter the size, but devote some freezer space to some ice bottles. Rabbits will love you for them.
Ears. A rabbit's AC system is their ears. You will notice those things get really warm when it's hot outside. Take some cool water and wet the inside and outside of them. This will cool them off quite a bit.
Do not handle your rabbits! Ever been in a bad mood? Don't want anyone to bother you? You should not handle your rabbits during the day. It will add to their stress they already are recieveing from the heat. I only handle mine in the weee morning or late at night. It's just not worth the risk.
Check, check and check again. You cannot know how your rabbits are doing unless you check on them. When I am home I check on them several times a day, probably every hour (esp in the hot part). When I am not home I ask of my family members to check on my animals for me. I am looking for rabbits that are wet in the nose, lethargic and unresponsive. I check to see how fast they are breathing.
My rabbits handle the heat well with a fan in my rabbitry and the fact my barn is under a whole lot of trees.