How to teach a horse upper-level dressage
Teaching a horse advanced dressage movements requires years of training and careful consideration of his balance, strength, suppleness, throughness, etc. Or, you know, go on a trail ride. This method is especially effective if you have an older, well-broke school master of a thoroughbred. One who is an arena flower to end all arena flowers, in fact.
For your first excursion on the trail, choose a sunny but windy day. The noise of the breeze through the leaves will terrify your arena flower relax the two of you the way those meditation CDs do. Ask for a calm, relaxed walk. With luck, what you will actually get will be a lovely passage. Stay relaxed and supple in the saddle, as at any moment the moving shadows may encourage your horse to attempt a half pass at the trot or canter.
Then turn a corner and find a moose blocking the road. You should experience, in succession: piaffe, a canter half-pirouette, and a marvelous extended canter. Do not attempt to maintain a strong contact here–this is the perfect time to return to basics and practice looping the rein at the horse to check his balance (think of all your hard work on those 20 meter stretchy circles!).
This is enough work for one day; you wouldn’t want to strain your horse with these new movements. Pick yourself up out of the gravel, dust yourself off, fetch your horse from the patch of grass he’s found to graze in, and head home.
When he’s had a few days to almost but not quite forget about the moose, recruit some friends to trail ride with you and view your horse’s new skills. Good riding partners: the green, just-broke mare, the “hottest” horse in the barn, and Spooks-At-Everything Dobbins. Gather together in the barn parking lot. As the other horses stand waiting for the signal to head out, your horse should begin demonstrating his new piaffe abilities.
As you walk out the farm gate, allow your horse to transition into passage. If he does not do this on his own, whisper “Moose” in his ear. You should feel an immediate elevation in his gait. When you are ready, encourage the other riders to trot. Your horse will begin practicing his collected canter. Stay loose in your seat and allow your legs to swing randomly; your horse may well begin showing off one-tempi changes. Reward him for his behavior–for example, as the riders in front of you begin to canter up a hill, you may wish to encourage your horse with a confident, “Ohmigod we’re going to diiiiieeeeeeeeee!” When you reach the top of the hill, he may be so proud of his accomplishment that he will again demonstrate airs above grounds.
As you’ve now demonstrated all his new movements, this would be a good time to collect your riding companions from the various ditches and hedges into which their excited horses have thrown them. If they seem inclined to grumble at you, point out the positive: their horses, too, show some talent at these advanced movements. Y’all will be showing Grand Prix by the end of the summer, for sure.
If, of course, you can catch your horses first.

RiderOne says 18 January 2007
*Snort!* Thanks for making up my mind about the trailriding issue.