If you've got lights in the barn, wait until after dark. It helps a lot. I've also found that putting on a wet t-shirt under my denim shirt helps keep me kool. Sometimes when I'm having to bushhog in the heat of the day, I'll wet myself all over with the hose before getting on the tractor. I'll be dry in an hour or so and I'll get off and go do it again.
I've been known to stick a wet towel in the freezer till it's almost frozen solid and wrap that around my shoulders on the back of my neck while mowing hmmmmm coool. :~D