Shaded places where they can dig into the ground and spread out (assuming the ground is dry) are best, plus of course plenty of ventilation. The ground, even just an inch down, is much cooler than the air temp, and of course the earth is the world's biggest heat sink! I know how impossible it is to keep green growing things alive in a run, but in the shade of a nice bush or shrub is a great place for a chicken to hide out the heat of the day.
Breed choices help too - large prominent combs, clean legs are good. Lightly feathered birds are good (Brahma, Cochin, Orps, not so much). Smaller birds are marginally better - the CornishX is not particularly heat tolerant (honestly, the CornishX is rather brittle, health wise) though they can manage with good care.
I'm of two minds regarding misters/water. Chickens can't sweat. If its 90+ degrees outside and 90%+ humidity (i.e. Florida), adding moisture to the air isn't going to help make things better. If, OTOH, its 105 degrees and 30% humidity (central TX, where I last lived), the combination of cool water plus evaporation will definitely drop the effective temp some.
People do put ice blocks in the drinking water, offer frozen treats - both good ways of dropping core temperature quickly. Electrolyte support, to help offset assumed extra water consumption also a good plan.
If you are lucky enough to have a huge water storage system (I have a 275 gallon tote,. fed by rainwater), it tends to stay close to the average daily temp because its such a huge heat sink - so when my nights are 70, and my days are 90, that's 80 degree water +/-, not the 100+ degrees of the 5 gallon blue bucket with nipples in the run. Likewise if you use a hose fed pressurized water system for your birds - it will be close to ground temp, even better than my rainwater tote.
