If you have shipping boots, it would be a good idea to use them. Fill a hay bag or net and hang it in the trailer for your horse. If he or she gets anxious or nervous in the trailer, a calming aid may be a good idea.
Bring several of your own clean buckets. If you can transport water with you, do. When traveling long distances, it's important to keep your horse hydrated. If your horse has problems drinking away from home, bring electrolytes.
Make sure you have a lot of hay – preferably what your horse has been eating. Bring any grains or other feeds that he or she regularly receives, along with any regular supplements. When I travel, I always grab a big bag of carrots and give them little snacks whenever I stop. You want to make this experience positive for them.
Make sure you travel with a veterinary kit. Be prepared for anything –*because now, at the most inconvenient time, is probably when it's going to happen. ;)
Before you go on this adventure, thoroughly bathe and groom your horses. Trust me, you're going to want them to look presentable! Also, make sure you're bringing along a grooming kit (emphasis on the hoof pick, here!).
Bed the trailer deeply with shavings (I've had good experiences with cedar, but pine or whatever is good, too!), and make sure that you clean out all wetness and droppings every time you stop so it doesn't get gross in there.
If you're traveling with the windows open, make sure you put a fly mask on. This will help protect him/her from bugs, rocks, etc.
If it's going to be hot, open some windows and make sure that there's good ventilation throughout.
Stop every few hours (if possible) to offer them water and, (circumstance willing) stretch their legs. Make sure you have a large amount of possible stops along the way –*that way if you don't make it to one on time, you have another! When your horse arrives at a "checkpoint" make sure he or she has plentiful water. Feed their regular dinner/breakfast (I'm taking grain or pelleted feeds here) around the usual time, so that their system won't be messed with too much. If you can, take them out of the trailer and let them eat their grain outside. They should have free choice hay at all times. If alfalfa will be too much for them, get some orchard or grass or oat hay for them to munch on.
Bring different weighted sheets/blankets. Since it's summer, chances are it won't be too cold, but weather is unpredictable and the key word in traveling is "prepared." It's not really necessary to put a sheet on all the time, only when it's cold. Also remember the factor of wind chill. But with the expectation of warmer weather, that will be a good thing!! ;) If it starts raining, close the windows –*rain can get pretty rough when traveling at high speeds!
Drive cautiously and carefully; don't jolt them around. Once again, you want this to be a pleasant experience for them! Remember that standing in the trailer is hard work, so make it as easy as possible.
Call ahead to your checkpoints (arrange these before you leave) to let them know when you will arrive (also when you're going to be earlier or later than expected). Don't be in any great hurry to get somewhere, just do your best and drive carefully!
Are you traveling alone or with someone else? Are you hauling one or more horses?
If you have any other questions, don't hesitate to ask!!