If you can't get your little darling to walk in a straight line, then require him to walk crooked. Keep him walking toward the left, toward the right, toward the left, toward the right, for a time spell. Then give him the choice to walk straight. If he is still going crooked, keep him walking crooked for a longer time spell. Be patient and soon enough he'll be walking straight.
Adventure Horse Riding in NYS. www.RideNYS.com