The best (read safest) way I've
found to get a horse used to a crupper is to do it in three steps.
First put them in a round pen loose and let them get used to the feel of
it. Next put them on a lunge
line and work them through all gaits. When they are comfortable with this,
put another lunge line on the crupper and really pull on the
crupper every few strides while the horse is working.
This seems to work very well with
most horses.
Also keep in mind to be effective
the must be fitted properly. Only enough room to fit a couple of
fingers under the tail.