What I've done is first teach them to stand still and what "woah" means. I practice with my young horses on the way to and from the round pen, but only for about 5 minutes at a time max. Once they learn woah, then I work with getting them to stretch and watch as they lean you can tell when they are just about to move that front leg. Tell them woah and maybe a jerk down (slight) on the rope. Keep the practice session w/ yearlings very short. They don't have much attention span before they are bored and you don't want them to feel sour about posing.