basically what dancingarabian said.
However you want to set your self up for success not failure. That comes from groundwork, Start teachiing your horse to go where you send him. On the ground. DOnt worry about the trailer yet. Get him in a round pen and do your normal lunge drills for warm up, I like to keep em at a steady walk, go left, go right, back up, come here, join up. I also prefer to keep a 12-15 foot line on them. Again your not sprinting or working on gaits just simple left right back come here. Next add stuff to the round pen. I use old truck tires and traffic cones, couple landscaping timbers, plastic tarp, then do the same stuff, send the horse over the obsticle, left right come here, same stuff. Send the horse, with you towards the center of the pen. work on that a few times a week and the horse gets used to the game. Next time you approach the trailer, the horse will look at it as another sending game. Stand off to the side of the door and use the same commands, motions to send the horse in the trailer, let him come up and sniff, being hesitant is ok, keep encouraging. But if he resists lifts his head and moves to the side or backs, back the snot out of him, then walk em back up to trailer, repeat, I dont do much more than an anoying tap tap tap on the withers, if they resist that they get backed up again. I might back em in circles or 50 feet across the yard. I guess I may be lucky or the fact I generally dont deal with butthead horses, but I havent ever had a horse take more than a couple round pen sessions, then one or two back ups to get them to load.
I do have to admit you add made me laugh. I got a picture of you lifting his feet up one at a time moving him up.