Repetition, repetition, repetition! That is all you can really do!
Take your horse into the arena without any other distractions. No other horses, etc.
Start from the ground. Lead him around, and periodically stop. Use verbal commands. Walk, whoa, stand. Say walk when you ask him to walk. Say whoa when you ask him to whoa. Say stand when you want him to stand still.
You'll have to start very small. Walk several steps, whoa, when he stops, say "stand". Let him think about it for a few seconds. If he moves around, say whoa, reinforce your cue with your body (gentle tug back on the halter), say stand, and you yourself stand stone still. When he stands for a few seconds, give him TONS of praise (not treats) and walk on. Lather, rinse, repeat. Over and over and over and over until he GETS IT. The horse needs to know that when you say whoa, you mean WHOA until you ask him to move. This won't happen all in one session, I'm sure, but you're reinforcing commands on the ground that you'll use in the saddle.
Don't get angry, or push him around or whatever if he's just fidgeting. If he's being dangerous, obviously correct him as needed, and go back to your exercise. So he's not super bored, throw in some backing, some trotting in hand, etc.
Move to the mounting block. OTTB's don't usually understand the concept of standing to be mounted when they are just out of their track life. Same exercise. Ask him to whoa, and stand nicely at the mounting block. When he moves, tell him to whoa, and stand. Reward him for standing even a short time, and continually ask him to stand for longer and longer.
Move to mounting work. Hop on, and ask him to stand still when you first get on. When you've got a pretty good stand on him, reward him, and get off. Move to the mounting block again (as he's surely fidgeted away), and repeat this process. Over and over and over and over.
When he'll stand calmly at the block and wait for you to ask him to move, move to saddle work. Ask him to walk several steps. Throw in a whoa (use your voice commands!!!). When he whoa's, ask him to stand. If he starts to move and get squirrely, ask him to whoa again, and tell him to stand. When he stands for a few seconds, reward him ("GOOD BOY!"), and ask him to walk. Repeat repeat repeat repeat repeat.
It will take awhile. He'll probably get mad. When he gets annoyed, ask him to do something else, move onto some trot work for a little bit. Ask him to whoa at some point, and start the cycle over. Always rewarding him for even a little time standing like a gentleman.
I hope you get the point. You use the same cues every time, on the ground, on the lunge, when mounting, and when riding. Eventually it'll be so engrained into his brain that you won't need to say the words.
I do this with all fidgety horses, especially when saddled. I'll stand there for 20 minutes at a time if the horse doesn't get mad. Whoa is whoa. I ask them to stand, they stand. If they move one foot, I'll reinforce the whoa by feeling their mouth again, and ask them to stand.
Repetition is all you need. Don't get bored with it and think you need to stop. You need to fix the habit, even though it isn't as fun as loping around.
Owner and head trainer of SE-Wisconsin Horse Care
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