Ask, then tell, then demand. As Sky mentioned, don't kick. Go back to the basics. You ask for more forward action with your hips and a squeeze (that's your "ask"). You don't give a second chance. Wait a couple seconds. If you don't get the response you want, you either make all your cues more intense or tickle with the crop (that's your "tell"). If you don't get the response you want, you demand (give a resounding whack with the crop like you really mean it). If he bucks when you do this, you'll need to think of something different for your "demand" step. When he moves forward, don't yank back on the reins. He's doing what you asked - don't punish him for it. Gradually rein him in to where you want him. The point is to teach him that if he doesn't listen to you the first time, things will get worse for him. He will learn that it's easier to do what you want in the first place.