If you go to mount your horse from a block or tree stump in the woods or picnic table or whatever, and he moves away from you as you are about to get on and you're ready to do the splits - this works well. First, from the ground, then once he knows it, from the saddle: use your crop and tap on his hip to move him over toward you (you will probably want to reach across and over his hip to get to the opposite hip ) or wherever you want him to go and as soon as he does it - at first, even the tiniest itty-bitty move, stop the tap. Do it again and again until he gets the idea.
The only way your darling knows he got right answer is if you stop the tap immediately when he moves the distance and direction you want him to do.
Once, he has it down on one hip side, start on other hip side from the ground.
Once he has that down, start teaching it while you are up on the saddle (this is fun). When he learns it under saddle, you can get him to wiggle his happy little butt side to side like there is no tomorrrow. And, he'll be so proud of himself !
Mary Dixon Adventure Horse Riding in NYS RideNYS.com