1. Make sure the trailer is parked on concrete. This means that if your horse does misbehave, spook, or spin, he is far more likely to slip and injure himself.
2. Put a Dually halter on your horse, but don't fit it properly. That way it will be less effective.
3. Don't attach the trailer to a vehicle, or put the feet down at the back or the front. This makes it far more likely that the trailer will tip as you are practising loading, and scare your horse off for good.
4. Each time your horse backs away from the trailer, or swings his quarters around to the side, give him a stroke and take him away from the trailer to try again. This rewards him for backing away or turning away, and makes that behaviour more likely to happen.
This is exactly the situation that I saw occurring as I arrived at a yard recently to physio a horse there. Thankfully, there were no accidents, and with help from others on the yard, the horse did eventually load successfully several times.
Loading is one of those behavioural problems that is almost always fixable with the right advice. Intelligent Horsemanship recommended Associate's are specialists in loading problem loaders, so if you have a horse that you struggle to load on to your trailer or lorry, why not get it right from the start and give one of them a call? You can find their details at www.intelligenthorsemanship.co.uk.