The aim is to concentrate on the front of the horse, and not the back, as many people often do. Of course, you should be working with your horse well ahead of that all-important day when you absolutely have to get your horse loaded and on the road on time. You might have to do a few practice runs before your horse feels really comfortable. Put a bridle on your horse with a leadrope attached at the chin ring and protection (stable boots, shipping boots or standing bandages). Be sure to wear gloves and use dressage whip.
Ideally, park your horsebox or lorry in an enclosed area: a schooling yard, riding ring, dry paddock, etc. This will make it easier to catch your horse if he gets loose but will also make sure your horse can practise in an area where he feels comfortable.
If you want to ensure your horse quickly understands the task, start by doing a leading exercise where you teach him the basics and establish trust. This will make the rest of the exercise much easier.
Next, warm up your horse's hind legs by having him move side to side. Reassure your horse when he's close to you and reward him when he does what you ask. Warming up his hind legs will help you check whether you are able to maintain control over your horse's position when he is facing you. Next, face your horse and ask him to walk forward by touching his shoulder with the dressage whip. These two simple learning exercises will be a great help. Repeat them until your horse is able to do them well.
Once you've checked that your horse follows you and does what you ask, stand next to the trailer or truck. With your back to the truck, stand facing your horse. Reassure him if he seems nervous, then do the exercise again: align his hind legs so he's straight, then, if you're far from the truck, ask him to come forward by touching his shoulder.
You can now climb into the trailer. Facing your horse, check that his hind legs are aligned (straighten them if necessary). Ask your horse to come forward so he can follow you inside. Do these steps over again one by one if necessary and reward your horse for each movement. This will help your horse understand and ease his nervousness. If your horse tries to back up once inside the trailer, do not pull too hard on the leadrope or he could hit his head by trying to pull away. Usually, by using the leadrope through the chin ring, you'll keep your horse from backing up very far. Have your horse exit the trailer as calmly as possible and start the exercise over again. Reassure your horse once he's inside.
Do the exercise several times until your horse understands he is in no danger in the trailer.