Posting or sitting, trot energetically down the long side-to the left, say, with your left leg at the girth and your right leg a hair behind the girth. Half-halt to rebalance your horse momentarily, and alert him that something's coming, by squeezing your outside right shoulder blade when his outside fore/inside hind come off the ground. (Hint: At the posting trot, that's when you're rising out of the saddle.) Softly flex your left wrist so you direct your inside rein toward your outside hip and ask for just enough bend in his jowl to bring his head and neck and shoulder a bit to the inside of the track.
As you bend your horse, keep the pressure and position of your outside leg the same but sit a little deeper on your inside seat bone-and imagine that your inside hip bone is shooting out a beam of light and directing all your energy down the rail, telling him, "Stay bent and go straight." If a friend were watching from the ground, she'd see him traveling on four very closely spaced tracks, with his front legs just barely to the inside of his hind (as opposed to the greater bend of shoulder-in, where he's traveling on three clear tracks-inside fore, outside fore and inside hind together, and outside hind).
If you feel your horse leaning against your left leg and falling in, sit even more deeply on your inside seat bone, give a strong half-halt on the outside rein, and direct your inside hip even more strongly down the rail. After two, three, four or more strides--when you feel him starting to understand that he doesn't turn until your inside leg relaxes and your outside aids finally tell him he can follow his nose-circle or turn. Straighten him when you return to the rail, and try the shoulder-fore again. When you can position him in shoulder-fore in one stride, and you feel that he's reliably waiting for you to tell him to turn, go on to the ten-meter half-circle.
This article first appeared in the January 1996 issue of Practical Horseman magazine.