I learn from people who do NOT have problems with their horses. I have learned that you need to practice trailering skills With your trailer, but there are ways to practice trailer loading skills withOUT your trailer. Slant load trailers have the same width as a human sized door and baffles horses that have never seen it before. So...you practice leading through human sized door in the barn, and backing through it. Much safer, because if something goes wrong you can drop the lead and then catch the horse. Once he has mastered it, your won't have that problem in the trailer, and it helps if your barn isn't well lit, like a dark trailer. Practice getting your horse to load himself by training him by voice to go into his stall with the lead draped over his back and then to face you to take off the halter. Also, even though Harley seems pretty well grounded, practice tying him up and look for excuses to tie him up and wait for you. I did that today. We just got 6 inches of rain in two days and have flood watches. My horses are stallbound, so I took them out one at a time and tied them to a post to wait for me to clean their stall. Buster Brown pawed and got his leg through the lead, above his knee and the lead snap was inches from his knee. Good training opportunity to teach him NOT to panic, and it WASN'T in a trailer. Every time something like this happens it builds trust, so if it happens in the trailer we won't have a horse explosion.
I have always used trailer ties with a quick release. AVOID the bungee cord trailer ties. If your horse pulls back they just overstretch and break and aren't worth it. The standard nylon ties work best. Long loopy leads in a trailer are an accident waiting to happen. Also, expecting your horse to keep his head up while travelling is asking for trouble. It's too tempting for him to drop his head down while in transit, and he could get stuck under a divider. Do NOT use a rope halter to tie to in the trailer. A standard nylon halter is better and one with a breakaway OR a leather crownpiece that buckles on both sides is the safest because it will breakaway in an emergency. http://www.statelinetack.com/item/we...-snap/E001017/
When I bought my QH/TQH cross who was my tough herd leader, and had been there and done practically everything, but I didn't know it, I enlisted help with somebody with a trailer. (I hadn't bought my first one yet.) She WISELY, took her time loading him, as if she was expecting trouble. He patiently waited, let her put in his two front feet and wait, and then load the back feet...and wait...and then lead forward to the front...and wait. Extra caution and EXTRA TIME is very important when you are learning this.
When you and your horse are pros you can try and break my record, 20 minutes FLAT, wrapping all legs and loading 4 horses!
Remember, a single horse travels in the front and left part of your trailer for the best balance. =D