I worked with a acting group of knights for a summer. They used horses that knew a number of tricks and I rode a mare that could rear on cue. Her cue was two part 1) preasure on the sides, but only from the heel and well behind the girth (further back than you'd press for normal cues) 2) draw back on the reins in a slightly downward manner.
I discovered this by accident one day while we were taking them swimming and she tried to kick at another horse, thus the leg pressure to move her away. (she refused to walk into the water and had to be backed, but would then swim around freely) Anyway, she never came off the ground more than 10 inches or so and it was always in a very purposeful manner, you knew you were rearing.