I taught my horse Red, and will be teaching my yearling, that when I throw the lead over his back and tap his hip with my hand and cluck, it means to load up. Now.
Right now I am using a bumper pull 3 horse slant with rear tack. Red will hop right into that 3rd and back stall no problem. I just give him his cue.
I taught this to him through lunging, but the main idea to convey is that when I tap his hip, it means to move forward. And you need to make sure your body language conveys where you want the horse to go. If you are too far in front of his shoulder, he'll go the opposite way from forward (because your body is "blocking" him) versus if you are standing behind his shoulder closer to the hip, which means go foward.
So I used lunging to teach him the "go forward" cue when I tap his hip. And then incorporated that "go forward" cue when I stand him at the door of the trailer, throw the lead over his back, and ask him to go forward into the trailer.
I always have all the drop down windows open on the trailer, so they can stick their head out. It gets them up into the stall farther and makes it easy to close the butt bar. Then its also easy to remove their lead rope, and clip on the trailer lead rope to tie.
When it's time to unload, I again open the windows so I can unhook the trailer tie. I always do that BEFORE opening the trailer and/or butt bar so they can't possibly fly backward and create a wreck while still tied.
Then I just pull backward on their halter (through the window for that back stall, because there's no room for a human) and say "back". And that's their cue to back out of the trailer. This you may need to teach by putting the horse in the front stall so that you have the room to walk up there with them to unload them, and back them off yourself, so they'll eventually know that's what you want them to do when you give them that cue in the back stall.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.