A method I use that works on all horses I have loaded...even abused and frightened new to trailering hroses...I will lead them to the trailer, if they go in and want back out, I let them. Sometimes we go in and out many, many times. But eventually they get tired of going in and out and calm down. Once they are calm, then I move to the next step of tying and closing the door, but never until they are calm. If they get nervous after I have tied, I untie and let them out if they want...and continue the process.
When they don't want to get in, I tap the rear end, and this really is a tap...constantly tapping with a lunch whip...not whipping, not hard, but enough to irritate....not by the legs, the top of the rear end... They want to move away from this irritating pest, but are not frightened by it. And it works every time..sometimes it takes a while and that is okay...I will never rush a new horse.....I have rescued, raised and trained over a hundred horses. It works.
I second what Ranelle said about lunging. If she backs out, calmly move her away from the trailer and lunge her in both directions for a few minutes. Then load her again. Each time she backs out, she should be lunged again. If she *starts* to back out and stops, ask her ONCE to step back in...if she doesn't, lunge her. She'll figure out that it's a lot less work to just stay in the #$%@ trailer than be lunged. :) We've used this on a couple of our horses who will load just fine, but every once in a while would get a pigheaded streak and plant their feet and say "no." They learned that "no" is not a word they're allowed to use. :)
Maybe some of you can help. I have a 4 yo Arab-cross mare who is a great endurance prospect in every way, except that she won't load in the trailer when it's time to travel. She loads when we aren't going anywhere, but rather practicing. How the heck does she know? I have been very careful to keep my energy level the same( traveling or practicing) and to show no sign of urgency, but she figures it out every time.
She goes in the trailer, just won't stay to allow the butt bar to be fastened. No way to have a second person help with loading.... have to find a way to do it solo. Using a Brenderup 2H straight load with a ramp. I lead her in, then when I try to go behind to fasten, she backs out calmly...over and over again. Freaks out and sets back if I try to tie her (only sets back if tied in trailer). She drives on the ground, but not into the trailer, so that's another hurdle here.
Advice would be appreciated. Just in case you're wondering, I am not an idiot with no experience. I know this is a ground manners and respect issue. We have great rapport except for trailer loading... I need some new ideas/advice. I rarely post b/c advice occasionally tends to be on the condescending side... I'll suffer through any amount of condecension to get some real help with this!