I think the thing most folks forget is to train the horse to load BEFORE you really need him to. In other words, if you know that 1 month or 6 months from now you will need to load him, start working with him TODAY. I spend maybe 15 mins/day teaching loading/unloading to my babies and by the time we need to go somewhere it's not a problem.
I lead them up to the trailer (which is scarey all by itself) and get them to pause there for like 5 secs. Then walk away and praise praise praise. Then walk back up and pause again. Reapeat until the horse will walk right up to the trailer and stop, never asking him to load. I have a ramp which I think makes it easier because my next move is to get 1 foot on the ramp and quit. Then 2 feet, then 3 then stand on the ramp. Once we're confident at this, then I let them go into the trailer and come right back out. Basically, they set the pace and determine what we accomlish that day. Anyway, after a week or 2 of this, they will load and unload very easily and rarely do I have a problem with a horse I've trained, getting on or off a trailer.