Also stick to what you want him to do! If you want him to turn left or right or whatever, you stick to it and don't let him get away with anythin.. If he starts goin in the middle then make him do a circle in the opposite direction. You really got to make sure that horse knows whose top mare, because if he thinks you are easy to get by then he will play on that and make your ride and everythin else miserable.
Be firm, and don't let him get away with anythin.