This hasn't been a problem in Vermont yet, where half the barn is still blanketed overnight and it's been raining fit to build an ark, but I did enjoy this SmartPak article: Cooling Out a Hot Horse.
At my first barn, when I was eight years old or so, we cooled out after lessons by walking down a fenceline, around the pole at the end, and back up the fenceline, a total trip of about 50 yards or so. At the starting point was a huge tub of water. We were to offer the horses water each time we reached the tub, and to keep walking until the horses spurned the water.
It's a simple if not ideal system. The horses were almost never worked hard enough to be breathing heavily, and they were all fit lesson horses anyway. I can't remember ever sponging or hosing off a sweaty horse.
Now I'm lucky to own a horse that isn't much of a sweater and cools down fairly easily. We always end our rides by walking for at least 10 minutes anyway, more for the muscle recovery than for actual cooling down strategies, though on rare occasions I've walked him longer than that when I feel he's warmer than normal.
He actually usually sweats and dehydrates more because of mental factors than physical ones - when I first started working with him, we'd spend 10 minutes in the indoor at a time, and all I would do would be to groom him slowly and gently, pick up a foot and put it down, and talk to him. He'd go back to his paddock after those sessions and drink and drink and drink. Like a nervous public speaker in front of a crowd of thousands - he'd get an equine form of cottonmouth.
Lately, I've been paying more attention to strength than to heat/dehydration. It's tough to find a flat surface to walk on so we've been doing more hill work than I had hoped for, and at the end of our trots (still 5 minutes) I am feeling just a teensy bit of wobbliness. I'm compensating by taking our overall progression more slowly and by making sure he has recovery time - he's getting tonight off, for instance. There's already a marked improvement in how eager he is to move out at the beginning of our rides each night.