Originally Posted by kitten_Val
If you are seriously looking into that 1) your horse should be capable of doing that, 2) you have to know the cues (meaning you have to be an advanced level rider), and 3) get a knowledgeable trainer to teach horse the piaffe. Although I don't want to sound discouraging, but it takes years for the dressage trainer to go to the level the horse is doing piaffe correctly (and you have to get all other basics in place first).
^Agreed. My horse and I are nowhere near ready to attempt piaffe or passage, but much of my research suggests that if the correct preliminary work has been done, and the horse is relaxed, supple, truly accepting contact and the aids, moving with impulsion, straightness, and collection (in the dressage sense), then teaching advanced movements like piaffe should be "relatively simple" - the groundwork has already been laid.
Of course, there is a difference between piaffe as a true and correct high level dressage movement and piaffe as more of a "circus trick." Most horses who learn it as a trick rather than as the culmination of years of mental and physical conditioning lack something in the foundation that makes the movement "true" in the competitive or classical dressage sense.
I'll second the caution on WikiHow - not only is the whole "wiki" conglomeration not a scholarly source, but some of the info and instructions are complete tripe... Finding a real dressage trainer is your best bet if you're interested in paiffe as a Dressage movement. It's one of those things that can be at best difficult for you and the horse to do without the right background and experience.