Here's how we do it:
Also, to keep them drinking enough, I offer them beet pulp-alfalfa cube soup in the trailer along with their water bucket. Sometimes I add carrots to the water bucket, to flavor the water and encourage them to play bob-for-carrots along the way. Yes, this makes a huge mess, but in addition to getting water in their systems, it gives them some long-staple fiber in their stomachs to absorb any trailer-stress stomach acid, which helps prevent uclers. Cleaning towels are cheap, Ulcerguard isn't.
I electrolyte the night before we trailer and after we get home. I don't electrolyte just before trailering, as giving the horses something to make them thirsty then putting them in a moving trailer where some horses won't drink, isn't really a good idea.
I am realistic with my expectations on hot days. I can't be angry if a horse isn't 100% when the temps are 100, that just isn't fair. And yes, sometimes we even stay home on hot days. Sometimes.
(yes, I know I haven't blogged in forever, I'll get an update blog up soon. Promise.)