When my wife had horses she tied reflective streamers, Pie pans, wind chimes and other stuff that would flitter flash and make noise in the wind all around the barns and arena. This got the horses used to movement and noises so that they wouldn't spook in the arena with people moving and the occasional dumb @$$ with a flash camera.
She also tied pie pans and tin cans to their saddles to condition them for the same reason.
Occasionally she sold a horse a horse never got over used to all the extra movement and was sold.