I purchased Clinton Anderson's Trouble Free Trailing DVD and it is worth every single penny. All
trailering issues come about because your horse is either 1) scared 2) lacks respect, or a combination of both.
I don't feed a horse treats to get them on the trailer. What happens when the day comes when they are not hungry?
I don't like to just feed them in the trailer. Because you aren't teaching them to load when YOU ask. You are teaching them to casually go on when they feel like it.
I don't like loading a horse with a butt rope or any sort of "emergency" device .... unless it is a true emergency (like a fire) and you must get that horse on the trailer right now. Those methods do not work if you want to load your horse for a lifetime.
Here's a snipet of CA's method from YouTube. I advise you to buy the actual DVD. It's awesome, and addresses other issues besides trailer loading that stem from the same problem.
His method has to do with "making the wrong thing hard, and making the right thing easy" but his method also has to do with getting your horse to have good ground manners.
You need to have 100% control of your horse's hips, shoulders, ribcage, and head before you can expect to put them into a trailer. If you can't control those body parts at any time, even if you are 10 feet away from the horse, forget about trying to force a 1200+ pound animal into a tin can.
Also. Do some searching on this forum. This topic of traile rloading comes up all the time!
Timing and consistency are crucial. Which is very hard to convey over the internet on a forum. If you can't find a trainer to help you in person, a DVD with video is the next best thing.