*People listen a little better than horses, set them up and watch for the horse to be correct with ears up and attentive expression.
*Take your time, horse respond to soft voices and quite movement, remember this and avoid your subjects literally running away.
*Step back and use a long lens, one challenge with a horse is avoiding lens distortion, while a close up of a horses nose may look cute, no one likes their nose or their horses to look long and big. A zoom lens will allow you to step back and zoom in avoiding lens distortion.
*Show love, people love their horses like people love their dogs. Set up poses and settings that allow that affection to shine.