Teach your horse the basics of flexion by using soft hands and smart techniques.
Also breezes, falling leaves, other living things, strange noises, something placed 2' away from where it normally is....
Only horse people get it...
Tips to keep your horse standing still while you get on | The American Quarter Horse Journal
Learn to Post the Trot for a More Comfortable Horseback Ride
How To Post the Trot
Problem: Your horse needs long-trotting for conditioning, but you never seem to do it enough. (Long-trotting, a key aerobic exercise for horses, involves moving the horse into an extended trot, covering more ground and raising his heart rate.)