Trailer-loading, most definitely. My family currently owns a 3-horse slant (which is for sale, btw) which has a mid tack. The horse area is big and open which is very nice for claustrophobic horses getting in, but not so good for horses that are not willing to stay in the trailer. And Royal was one of those horses.
I tried for five years to get him to stay in the trailer without being forced. But nothing worked. He'd hop right in... and immediately turn around and try to come back out. I tried loading and unloading until he realized it was no big deal, but that just ingrained the habit further. Asking him to keep his nose straight with my carrot stick only lead to panic. And he absolutely hates to be lead into the trailer. So, the way it worked for those five years was he'd get in the trailer, turn around, I step in and ask him to turn back around to the front stall, get in position, tie him, and close the divider. It was always very tense, but I had no idea how to make it better. I couldn't spend untold hours loading and unloading him, nor could I leave the trailer in his pasture for him to explore on his own.
So, I got my clicker. And I started the clicker training process.
Step 1: Load the clicker. This just means you establish the click as a good thing. You click and immediately give the horse a treat. Click/treat, click/treat, click/treat, click/treat, all in rapid succession. I used Royal's favorite: Winnie's Cookies for this, but I also had small pieces of carrot.
Step 2: Start the puzzle. I lead Royal over to the trailer with the door open. He looked at the trailer, I clicked and gave him a treat. He looked again, click/treat. I ask him to step towards the trailer, he did, click/treat. He put his nose on the trailer floor, click-treat. Now we're getting somewhere.
Step 3: Shape the behavior. Then I only started trying to get him to load himself by getting progressively more particular when I would re-send him. First, I started only clicking and treating if he put a hoof in the trailer. Then 2. Then putting those two a little further in. Then trying to get his hind legs in. Then the Jackpot (two whole cookies) came when all four feet went in and, for the first time ever, he didn't immediately turn around. This was HUGE. Click/treat for that five times in a row, then asked him to go a little more into the trailer. Then all the way back. Then put his nose on the tie ring. Then stay there for a while. Then wait for me to walk up to him. Then for calmness while being tied. Calmness while being untied. Calmness walking out.
Everything was broken into tiny steps, with lots of retries, re-sends, and some guidance in between. All in all, the whole process took about an hour, and it's been reasonably stress-free since. I think some of his stress has to do with the fact that the trailer is a bit small for him (it's a 7'x7') but he's a lot better now. If I am able to get a new trailer this year, this is the process I'm going to follow, because it works for us.