Loading a horse into a trailer is a test of how accurately the horse responds to the step cue you put on him by heeding. Trailer loading isn't a separate skill you and your horse must learn. It's just applying the step cue you taught your horse with basic heeding to a specific task. When the horse understands your step as an cue, meaning he is to follow each of your steps with a step of his own, you can use that cue to ask him to enter the trailer with you.
Horses that have had bad experiences remember trailers as scary things. If your horse is scared of the trailer because of previous bad experiences you must treat it like a new piece of equipment. All new equipment must be presented slowly and in a calm working environment. You reintroduce the trailer slowly, in a relaxed manner, with rhythmic use of your heeding cues.
If your horse is very afraid to even go near the trailer, do calm and familiar things beside it. For example, you can heed the horse in large circles next to the trailer because the circle is a familiar shape and you always want to be directing the horse what to do.
What you do with a horse that is terribly afraid of the trailer because of previous bad experiences is just calmly get him working on your step cue and walk him up to the trailer. The truly scared horse has a tendency to rock back on its haunches with his attention locked on the trailer as he approaches it. Gradually, you'll heed him closer and closer till you get him right up to the ramp. Then he'll literally try to go up the ramp into the trailer on his toes as he walks in beside you. So take some familiar-smelling bedding from his stall and cover the ramp and trailer floor about 6 inches deep so there's no chance he'll slip and slide when he first tries to tiptoe inside.
When you are reintroducing trailers as good things, you don't want the horse to be afraid to escape. So if he wants to escape, you let him. Stop at the point where he begins to hesitate and acts like he wants to escape. Let him investigate. Make sure you reinforce your friendship with the horse. Groom him, scratch him, talk nice and don't apply any loud pressures. Do this over and over until the scary spot gets closer and closer to the trailer.
Heeding makes the horse feel safest at your shoulder. A lot of times, the horse will be perfectly willing to stay next to your shoulder but he won't be relaxed and will therefore want to escape (with you) when things start getting scary. So you must achieve rhythm and relaxation during each stage of introducing this "new" trailering equipment. Most horses will not be this fearful, however. Horses that are very afraid are usually horses who have been beaten into a trailer before and are afraid that they will be beaten again. You must be very patient and calm with these horses and give them time to trust you.
If your horse gets excited, stay as close to the shoulder as you can. Your body there will calm the horse. And if the horse gets scared, it is even more imperative that you remain in position at the shoulder. Leaving the shoulder in a time of crisis is like the pilot of an airplane running to the back because he thinks the plane is going to crash. You have to consistently stay in position alongside the horse's shoulder so that the horse starts to realize that he can rely on you whenever you're in that position.
You must make your horse calm at all times when he is next to you so that if he gets scared, he comes to you. If your horse is AFRAID of the trailer, you must stay at the shoulder so that he has somewhere safe to be and before you know it, he is trusting you and walking with you into the trailer.
You must give him all the time he needs to get comfortable with the trailer. Do not force the issue. Let him check it out. Give him time to be curious. Keep him paying attention to the trailer and to you. Don't let his head go to the outside or behind you. If he backs up, stay at his shoulder, and ask for back. Make it your idea. Let him calm down by giving him something to do that he already understands and can be successful doing. Then walk forward again. Show the horse what to do. By backing and walking forward again behind the trailer, the area that the horse is comfortable in will get larger and larger until he is also comfortable walking into the trailer.
Getting the horse into the trailer is not the big goal. The big goal is getting the horse to willingly go with you anywhere, to follow your step aid accurately and willingly. If the horse will not go somewhere with you, you must fine tune the heeding and earn more trust.
Never hit a horse that's afraid of the trailer with your whip to get him to go in. The object is to get the horse to want to get in the trailer, not to trap him in the trailer. He should go in because he trusts you and because he feels safe next to your shoulder, not because you have whipped him in. But if you start a fight or force him into the trailer, you will only make the situation worse.
So be patient and be his friend. Keep going back and heeding around the trailer, maybe even do some lunging near the trailer. Keep the situation calm, keep the horse working in rhythm and relaxation until he realizes that when he is with you, the trailer is not scary.