People think because we're a mile high, we don't get heat, but this year we've been driving in 110 degrees. Not hard, but as was said, you cannot control weather at events. I have a Betathane harness so sometimes I will hose PJ off while in harness. We might be working out down in our obstacle yard, have a nice hose off, then go off down the road.
I also feed electrolytes, although it took me awhile to find a great combination. Once I did, though, both horses really perked up. I frequently take some sort of sports drink with me when I drive (yes, I share it).
Last week, I actually trace clipped PJ! He is SOOO much happier! We have temperature changes of 50 degrees in a day, and a couple times 60! It will be 100 - 110 by 3PM and 45 at 3AM. Very sucky.