To improve your horse's sidepass, use the fence to act as a barrier instead of using bit pressure, thereby, allowing the horse to feel the communication from the riders leg.  Begin by walking parallel to the rail, guide the horses head toward the rail by applying inside rein pressure, place the leg slightly back from the normal riding position to communicate to the horse to move their hindquarters away from the leg. 

      To ride a left sidepass, apply the left rein to the neck to guide the head and the right leg will be used to push the hindquarters to the left.
TRAINING TIP FOR SIDEPASSING
Improve Your Horse's Sidepass