Join Date: Oct 2009
Location: Northern Nevada
I'm far from being an expert, but I have gone through this with my horse. She was very hard to load when I finally started riding her this spring, I think because her few trailer rides had all had unpleasant consequences, like vet visits & surgery. (Indeed, IIRC she almost always was tranquilized.) Now she's very easy to load. Here's how we went about it, for what it's worth...
As others have said, make sure the horse is good at being led by you before you start.
What we did to start was to hook a lunge line to her halter in addition to the lead rope. That went through a loop in the trailer (it's a two horse, slant load), and was held by the second person, well outside. The idea is to keep enough pressure on it so the horse can't turn quickly and get out, trampling you in the process. The second person may also have a "carrot stick" for the occasional butt smack.
We began leading her into the trailer. At first she would only get her front feet in before she wanted to turn. We kept her from turning for a few seconds, praising her, then let her turn to come out. Eventually she'd get four feet in before wanting to turn. Now when she wants to get out, make her stop and wait a few seconds at the edge (facing out) before telling her to go ahead. Eventually she gets to the point where she will stand in position, and you can clip in a tie rope and close the divider panel. Of course praise and treats are given for each successful step. And what I found very important was that _I_ had to remain calm and treat it as a normal, everyday thing. If I got nervous or hasty (and I am very much a novice), she'd pick up on it and want to turn and get out.
What else I think was important was that every trailer ride meant she either got to go for an interesting trail ride, or to come home, so she came to associate the trailer with good things instead of bad things. Now she loads up like a champ, without any fuss or bother at all. But it did take at least two months of 1-2 rides per week to get there. I honestly don't think it's something you're going to manage in a day, especially by yourself.