Some good ideas here.
The main cautions and suggestions I'd offer (some of which have been floated in previous posts) are):
- don't mess with the horse and trailer unless the trailer is hitched to the truck.
- if you have a GUARANTEED GOOD-LOADING horse, it is very useful to hold, pen or (safely) tie the pupil nearby to watch the good-loading horse go on and off a number of times, including watching him/her being given a treat once fully loaded in the trailer. Do not do this if there is the slightest question about whether the demonstration horse will sail right on -- you don't want to be demonstrating hesitation or misbehavior!
- work on getting the horse very, very good at going forward and backward on cue *on the ground* first, then "happen to find yourself" doing it into the trailer.
- don't try to go all the way in, in one go - stop partway, with YOU being the one who says stop, and back the horse calmly back out on YOUR say-so.
If the horse stops on its own, DO NOT BACK OUT at that point -- ask for, and obtain, at least one more step forwards before asking him to stop and reverse.
- I am not a big one for using treats to lure a horse into a trailer, unless you've got an essential vet clinic appointment twenty minutes ago and just *have* to get him loaded. All too many horses figure out how to 'work the system' and score treats without ever actually fully loading. OTOH I am a big supporter of waiting til the horse is loaded and then picking up a small bucket with something tasty in the bottom and hangnig it for the horse to enjoy.
- neither rush nor delay snapping the butt bar or butt chain. Try to do it too fast, like *as* the horse is stepping forward the final bit of the way, and they often get spooked and come rushing back out, which is not only unproductive but apt to result in injuries (not the least to whomever was trying to hook the butt bar!). OTOH leaving a horse in the trailer *without* the butt bar done up is just asking for him to ooze backwards to the end of the leadrope and either break loose or panic.
- once the butt bar is attached, close the doors/ramp right away, again to remove potential for excitement and vet bills and emergency room visits.
That said, you can also get horses into trailers on a fairly regular basis by chivvying them in with brooms or leadropes or pool noodles or whatever, or by linking hands behind the rump and physically PUTTING the horse in. The thing is, even if you have fairly good natural-born tact for these methods -- some do, many don't -- they still have more of a likelihood of "fireworks" and a bad experience for the horse. So if you have time -- and it sounds like you do -- I think it is well, well worth approaching it the way you are doing, as a training issue so the horse *learns* what to do and do it reliably.
Good luck, have fun,
Pat