Provide lots of shade options.
Breezes - run fans they that can choose to stand near and can choose to roost near (for roosting, not blowing directly on them, but circulating air in the room).
Icy cold treats - watermelon etc. I put wedges of watermelon in the freezer for 20-30 minutes before serving and they love it on a hot day. Cantelope, berries - many fruits are enjoyed by chickens.
Cool water available at all times - this may require putting chunks of ice in water bowls and then changing when melted and too warm.
For really grizzly days a bit of electroltyes in their water or drizzled on their cold fruit (can use pedialtye in a pinch - must be kept refrigerated - or use one of the electrolyte recipes found here on BYC)
No heavy foods in the heat of day.
My birds have never ever appreciated being wet tho I hear some do. There are misters that bring down the air temp - you can search here on BYC for more info.
JJ