What he said. Water, shade, and ventilation. Lots of water. Water in the shade, where they hang out. All the places they hang out. Layers of shade also is better than just shade. For instance, a tarp over an area that is already shaded by a tree.
And plenty of ventilation. Sure, chickens don't sweat so a breeze doesn't cool them the way it does us, but it does carry away warmer air and humidity, hopefully. If you have a fan going and it isn't doing that, then you may as well turn it off. Many of us in the south have coops with at least one completely open wall as well. The south wall is the one to leave open, though you do cover it with hardware cloth or welded wire. But even better, the south and the east walls open. Prevailing summer breezes are from the south and east in the USA and the winter, bitter cold ones are from the north and west. If you live in the city, you might have houses too close together or solid fences that will mess up air circulation. I'm lucky enough to have a good sized city lot with that "hurricane" type wire fencing so get plenty breezes.
People suggest freezing water in soda bottles and milk jugs to put out but I dont' do that. I have a shallow 3" pan that is a few feet long and wide that I fill with about 2" of water for them to wade in. This is set in the shade and they really do cool off in that thing.
My fluffy birds have survived temperatures and high humidity that has killed many a bird in my area, close the the Gulf Coast in Texas. Summers are normally around 100 degrees in August and humidity around 70%. But last year we were more often at 110. So far, not a one lost to the heat.
