I don't really do anything, except stay as balanced at possible so I don't throw the horse off. Horses naturally change leads, and a lot of the time don't when ridden because the rider is throwing their balance off. Try to focus on sitting in the center, don't lean, and maybe bend their head to the outside a little. When practicing with my mare, I'll do figure eights and when I want her to do a flying change, I act like I'm gonnna keep circling the way I'm going, then at the last moment I turn the other way to circle, making her change her lead. I do this exercise for a few minutes every ride.. It helps her get in shape and lets her practice her changes.
“Have fun - Stay on top.”
"There's nothing like sixteen hands between your legs"