"Here, Kitty Kitty..."
Unless they have been educated, new horse owners often think a horse is like a cat or dog. They figure if they tap their thighs and say, "C'mon,...C'mon,...C'mon..." horse'll will simply jump right in trailer like a happy dog or cat.
"Using Food As Bait"
Putting hay, grain, apples, or whatever at front of trailer to tempt a horse to step in and eat almost never works. If it did, it would be a fluke. I've seen horses lean forward to try and eat food but wouldn't step into trailer if their life depended on it.
"Forgetting To Hook The Trailer To The Truck"
Don't forget to hitch trailer to truck before getting a horse to go in trailer. If a horse steps into a trailer that moves around unforgivably, you will have a harder time getting that horse in later. He'll remember it - especially if this is horse's first time.
"The Classic Tug Of War"
Here's scene. Man (or woman) pulls lead rope to desperately drag their horse into trailer. Horse weighs 10 times more than man or woman and has far more strength than man or woman. Final score of this battle is: Human - Zero...Horse - Won
"Going Trail Riding Before Horse Is Good At Loading In A Trailer"
I've seen it time and time again. People go trail riding and when ride is over horse won't get back in trailer. Amusingly, horse owner comments, "Dang horse, he got in their last month". Remember to get your horse to practice this so it gets fixed on his brain.
It seems there will always be at least once a horse owner cannot load his horse into a trailer. But secret is to teach a horse sending signals so he knows what you want him to do. It's partly how man and horse communicate.