When it was 100 degrees here in Ohio last summer, I ran the hose into the dirt under their shade trees and made it all gloriously muddy. They dug holes into the cool soil and splashed in the puddles and I didn't lose a single hen.
Some people just flat-out hosed their birds off, but I don't agree with that since it causes the feathers to lose their insulating qualities and insulation can keep an animal cool as well as warm.
I would definitely put fans in the chicken coop to get the air moving. Some people make swamp coolers with wet burlap sacks to cool things down if it isn't already super humid out.
Your mister idea might work on super hot days, too. Just remember that when the humidity is too high, the water can't evaporate and will leave them hotter than ever once it warms up.