Ziggy's advice is straight on as far as how I train or retrain a horse to load; you HAVE to make going into the trailer the 'easy' thing for the horse to do, while him not wanting to go in, becomes difficult, as it means work, work, work!
My last horse was terrified of the trailer because of one bad experience; it's sad because the first time I had him in a trailer he loaded in about 5 minutes, but then the people who I was boarding him with (for the short time I kept him there) decided they could load him themselves, even though he didn't know, or trust them...they wound up pulling him in with a buttrope; I was furious, because I had told them I would load him, and I would be there at such and such a time to do so. At any rate once I started working him to load, the keeping his feet moving method worked fabulous; it did still take roughly an hour of work for him to realize that actually getting in the trailer was what was desired, and once he realized that, he never looked back!
Make the desired behavior easy, and make sure the undesirable behavior is difficult. He will catch on...
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."