Keeping them shaded, hydrated, and having fans/misters available is about all you can do for them. Beyond that they just have to live with the heat like the rest of us.
Of course when its very hot out you should avoid working them or turning them out with friends who will like to run a lot, but most adult horses are sensible enough to make use of shade, breezes and water to keep themselves cool and aren't likely to exert themselves when its too warm anyway. Some people have success putting wading pools out for the horses to play with (though some horses would rather you just left the hose on in a corner somewhere and made them a mud pit to wallow in LOL) and on very hot days I will usually put frozen 2 liter bottles of water in the water buckets so the water stays cooler and more refreshing.
One thing to remember about hosing down a horse is that on dark horses especially that water on their bodies can actually heat up FAST and make them HOTTER than if they were just sweating (which has a chance of offering some cooling by evaporation). Give them shade if they are wet down!