What's the clearest, simplest way to introduce your horse to the idea of moving sideways? Teach him the turn on the forehand.
From the halt, you'll use this schooling movement to teach your horse to respond to your direct inside leg pressure by moving his haunches around his forehand in an arc. He'll do this by crossing his inside hind leg diagonally forward in front of his outside hind. Once he masters this skill, he'll be better able to take on such show ring movements as leg yield and shoulder-in, and you're going to find that your control over the shape of turns, circles, and figures is much improved.
Find a spot near the center of your area where you and your horse have plenty of elbow room to make a turn. You're going to move him sideways off your left leg as I talk you through the pattern.
First, check your position. Make sure you're sitting balanced on both seat bones, with a tall upper body and your weight dropping down into your heels. Because you're going to move sideways to the right, ask for a slight inside, left flexion by turning the knuckles of your left hand toward your right hip. At the same time, sink a penny's worth more weight onto your left seat bone and down into your left leg.
Slightly soften the contact on your outside right rein to give him room to turn to the right, and allow your right leg to lie passively on his side--ready to be a wall that keeps his haunches from overreacting and swinging wide, or taking more than one step at a time.
Bring your left inside leg back just a little bit behind the girth and squeeze and press with your calf to ask your horse to move his haunches one step to the right in an arc around his forehand. As soon as he starts to take that first step with his inside left hind, tell him that's all the sideways movement you want. "Catch" his foot in mid-air by simultaneously relaxing your inside left leg and half-halting on your outside right rein. Your horse should respond by putting his foot down and completing the single stride of turn by squaring up his other feet.
Because this is very much an obedience exercise, stand at the halt and let your horse relax and settle. You always want a prompt response to your leg aid, but you don't just want him spinning around his forehand haphazardly. After three to five seconds, ask for another step, repeating the sequence no more than one or two more times. Then give him a long rein and do some free walk around the arena. Gather up your reins, work your way back to the center and start over again, this time asking for a turn on the forehand to the left.
When you and your horse can execute this movement consistently in both directions, you'll be better equipped to enjoy riding the gymnastic pattern below that I teach you in "Counterbend On a Serpentine" in the November 2005 issue of Practical Horseman.
The serpentine is the 12th pattern in this series. You prepare to ride the pattern by schooling your horse in a turn on the forehand, this time flexing him softly to one side and asking him to move his haunches around his forehand in the direction of his flexion and bend.