I'm catching up, ok?
Most of us have been somewhere with our horse, whether its camping, a trail ride, a horse show or just moving them to another barn. Like most things with horses, sometimes success is just a measure of trial and error. What is your best tip for traveling with horses?
Lots of other people have answered to make a list and check it twice. I do that. I even have a separate sub-list for my trailer-specific first aid kit. To describe it as obsessive would not be doing it justice.
However, here is my absolute #1 tip for travelling with horses: bring way, way more water than you think you will need, and bring it from home.
There are many reasons for this, so I'll break it down.
First, horses can become suddenly picky in the weirdest of ways. Having water that they're used to, from home, diminishes the chances that they'll go off drinking.
Second, never, ever, ever plan on water being available where you're going, even at shows. Biosecurity is a real thing. If you arrive at a show, and the only way to get water is out of a communal trough, DO NOT USE IT. Don't be that horrible example who brings strangles back to your home barn. (I am also fairly neurotic about not letting Tristan graze if trailer parking is in a strange pasture, but I know that's a little above and beyond.) Bringing your own water helps neatly avoid this problem. If there's a hose/spigot available, and you can fill up separately from the main trough that everyone and their cousin has used for their horse, then that's a bonus, and fill up your empty containers before you go home, just in case.
Corollary to this: don't be that person who waltzes up to the big trough and lets their horse drink right out of it. You are the Pony Club poster child for thoughtless horse owner. I'm serious about this. Don't do it. If you must, dunk a clean bucket in to fill up.
Third, you never know how long you're going to be stuck on a trailer. "Oh, it was only a short ride to the trailhead!" becomes a 4 hour wait in bumper-to-bumper traffic on a hot summer day. If you didn't bring enough water, or didn't fill all your empty buckets from the (safe!) hose before you left, then you can't pull over and offer your horse water and you are going to really, really regret your lack of preparation. I've been hauling horses at the tail end of what was supposed to be a short drive and sat for hours and hours in stopped traffic when a horrible accident closed the entire highway. I was very glad I could have my companion jump out and offer water to the horses.
I never hitch up the trailer without somewhere between 20 and 40 gallons of water, depending on how many horses I'm hauling. I usually fill up four 5 gallon buckets, and then I have a few 5 gallon gas containers that I fill up with water as well. I dump and refill them regularly if I haven't used them up. I scrub them and let them dry in the sun if they show the slightest hint of slime. (I also have 3-5 possible buckets to use for sponging or drinking for horses, so they're not drinking out of those buckets.) I covet one of those big water tanks that tucks under a gooseneck or in a tack room. Someday!
If you're hauling with someone else who doesn't have adequate water buckets in their rig, then it's your responsibility to provide water for your horse and, if you're a thoughtful person, for the other horse as well.
So, there's my lecture/advice. Water: don't leave home without it.