My neighbor is a retired trainer who does horse transport for a living. (snip)
It's the same method many have mentioned here. We use a regual halter and lead, bt you can use a rope halter. Use a long whip (like a longe whip but just the shaft part) so that you can tap the horse on the hip. Make sure you can rub the whip all over the horse so that the horse is not afraid of it before startting any trailer work. Walk the horse up calmly. When the horse stops, stay calm and give the horse a moment. As long as the horse is facing the trailer and paying attention, let them investigate and figure it out. If they are inattentive, put some pressure on the shank and tap the hip. As soon as the horse shifts forward, release the pressure and stop tapping. It is okay if the horse swings from side to side as long as they are paying attention and facing the trailer. They can step in from the side of the ramp and be fine. We usually have the divider tied to the side or taken out for a bad loader. It's not magic, and I'm sure a really confirmed bad loader would need a pro like my neighbor so that things to do not escalate and because the pro has better timing. It's okay for the horse to back off of the ramp and the trailer as long as he comes forward again. It usually does not take long at all (under an hour in the many, many I have seen my husband and him do), and it is not a scary experience at all for the horse.
I can vouch for the methods used by witherbee's trainer neighbor, as I had him come to my farm to help with a problem loader (this horse had fear issues due to a bad trailer loading incident at the vet's office). I had spent months getting the gelding to be good about loading, and had success using the above methods, my issue was getting him to stay on the trailer once he was on. I could not get the butt bar or ramp up.
The trainer came over, and evaluated the situation and confirmed that the horse was still phobic about the trailer itself, even after standing in it with me for 30 minutes, he was still not relaxed or bored, and any attempt to close him in caused panic on his part.
Based on suggestions from the trainer, I was working on that issue, up until the horse became sick and died of chronic kidney failure. I think he would have come around with enough time to work on that last issue.
There are friends and faces that may be forgotten, but there are horses that never will be. - Andy Adams
I agree with 20 min a day. We use to give the one mare her feed once she was on the trailer. After the 4th day she would "hop" on. It seems like you need 2 people to be effective. I would also do the chain under the lip theory. What is she doing? Is she getting nasty, rearing or just side stepping?