Preventions is the best method but knowing the symptoms and what to do is also key. Just as a note, rabbits with thinner coats and longer ears fair better in the heat.
- Hot ears, lots of blood is flowing try to cool off their body.
- Panting and flared nostrils
- Wet nose (not snot, just moisture) and sometimes sneezing
- Head tilted up and/or back
- Drooling or slobbering
- Rapid shallow breathing, difficulty breathing or gasping for air
Cool your rabbit slowly. If they are in the first stages, take them to a cooler area and wipe with ears and feet with rubbing alcohol or cool water. Try to do the preventative measure to continue to bring down their temperature. If in the progressive stages immerse them up to their neck in tepid water or lay a cool wet towel over their body and rub to soak the water to their skin. Do not take them into an air conditioned area as that is too extreme of a change and can put them into shock. Also don't use cold or icy water, for the same reason.
- No direct sunlight, shaded areas are great
- Frozen tiles for them to lay on (marble is great as it stays cool no matter what)
- Frozen soda bottles in their cage (they will lay on or next to it to cool off)
- Wet towel hung over the side of their cage (just don't have it dripping on the rabbit)
- Fans and misters near them but not directly on them (helps cool down their environment)
- Insulation boards over their hutch can help keep temps down
- Brush your rabbits to remove excess hair
Here is another link with more information: