Loading a horse into a trailer is not a separate skill that horses and their handlers need to learn. Loading is simply a response to the step cue you've taught your horse through heeding. The horse that has learned to stay at your shoulder, trust your consistency and trust that he's got your full attention whenever you're with him. He has learned that your step is a cue for him to take a step. So he will match you step for step and walk right into the trailer. The step cues are the same and the horse's response to them should be the same as if you were asking him to walk down the barn aisle or into an arena. When the horse understands heeding, walking into a trailer is just one step away from what he already knows.
When people have trouble loading their horses its usually because they didn't introduce the trailer properly in the first place. Horses that have been forced into trailers or had some other bad trailering experience get afraid of them. Other horses are not afraid. They have just decided that they are not going to get in the trailer and are simply being disobedient.
If you have a horse who is just being evasive and disobedient, the tactics are a bit different than those you use to reintroduce the scared horse to a trailer. But you cannot lose your temper or start fighting. Disobedient horses are spoiling for a fight. Remember that a horse can only get the energy for a fight from the person who's fighting with him. If you're not fighting with the horse, he'll quit.
A horse can stand physical pressure for a long time but it can't stand mental pressure very well at all. If the horse is constantly made to pay attention to the trailer and to you and is constantly prevented from fighting, it will eventually just walk in with you. After all, you didn't let it have any fun by fighting with you.
Your first job is to get the horse's attention and keep his attention. The disobedient horse will try to change the subject by fighting with you or turning his head away or digging in before he reaches the trailer so you have to keep him paying attention to your step cue and any aids you use to reinforce it. Don't let him get away with changing the subject by fighting. I repeat, do not argue or fight.
You get the horse's attention and enforce your control by giving the horse no choice but to stay next to your shoulder. You stay behind the trailer and heed. You back, halt, walk forward, halt. You must use definite step cues. If the horse doesn't pay attention to them reinforce them with your whip aid. Tap on the hindquarters to reinforce walking forward. To ask for a halt, stop stepping. If the horse ignores that cue, turn your body parallel to the horse to cue for the halt and use your whip to block his chest. Push the handle of the whip on his chest at the front of his shoulder as an aid to reinforce your step toward his hindquarters to back the horse.
Keep the horse working. Go back and forward and back and forward, stopping closer to the trailer every time until the horse walks in with you. Never take the mental pressure off. It only takes one second of rest for the disobedient horse to build up the energy to try evading your cues and reinforcing aids again.
If the horse refuses to do what you ask it, ask it to do something else. For example, if the horse will not walk forward to the trailer and wants to back away or run to either side, then ask it to back. Ask it to do turns. Ask it to back, then walk forward. Back, forward, back forward. Get the horse's full attention back on you by constantly giving him something to do. Do it quietly without fighting or forcing.
When you feel the horse is paying attention and you have control, ask the horse to walk into the trailer. You will probably need to reinforce your step cue with a whip tap on the hindquarters to get the horse into the trailer. The timing of the whip aid is crucial. The tap must come just as the horse is deciding whether or not to take the first step into the trailer. If the horse does not listen to the whip aid the first time, don't keep tapping. Go back to reinforcing your step cue with a lot of definite heeding. Heed right behind the trailer, work for accuracy, and keep up the mental pressure. Then ask the horse to go in again.
Be sure that the horse understands what you want. Keep in mind that the obedience you want is about the step. When you use a reinforcing aid, you are reinforcing in the horse's mind that he must obey your step cue. You are not reinforcing the issue of entering the trailer. If the horse will not move forward and has stopped paying attention to your cues, you must strengthen the horse's understanding of the cue and of the response you want (which is to follow your EVERY step). Or else you need to slow down and regain relaxation by going back to something the horse understands such as lunging. If your horse likes to fight you about the issue of the trailer, spend his energy lunging [behind the trailer] before you even ask him to walk into the trailer. Finish with the trailer as the last lesson for the day.
The object is to use your step cues to get the horse to respond in a certain way whenever and wherever you want. So if you fight with the horse and manage to get him into the trailer the first day, you have accomplished nothing. You have accomplished something when the horse responds consistently to your cues.
There are a lot of people paying a lot of money going to clinics hoping to learn some mystical technique to put them in control of their horse. They think that a "real" horse person can just walk into a barn, take any horse, and go right to doing whatever they want with it. But there isn't a real horse person I know of who would ever even try to do that. Real horse people know that control over a horse comes from earning that horse's respect and trust. You earn that by always telling them what to do in a calm and horse logical way. Every new thing you ask is just one step away from what the horse already knows.