If you have a round pen, practice falling off. Have a halter with a long lead in your hand, and start with a simple jump off. Progress to jumping off and crouching on the ground, then jumping off while holding onto a rain slicker or something. Be careful, don't do the slicker if he's not already ok with being ridden with one.
This is a topic being discussed on a local BB due to the exact situation you're discussing - that horse has been missing for weeks now, so there's a lot of discussion of training techniques to avoid this nightmare.
More good things to work on: practice dragging an object on the ground, and, simultaneously, woahing off of voice. If you get caught up in the stirrup, that horse better stop when you tell him!
Coming to yelling and clapping. If a horse gets loose, searchers are going to be making a lot of noise - yelling, whistling, rustling bushes. Teach your horse to come to lots of noise, not run away from it. Every time you feed, clap and yell, even if they're already at the feed buckets. Practice out in the pasture - start off with treat rewards, but try to wean off to verbal praise as fast as possible, without backsliding in training.
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