How to stop a horse rearing

There are so many crazy ideas on how to stop a horse rearing, and most of them only make things worse. Jamie and Gareth discuss some techniques that actually work, showing them on a horse that hasn't learned to rear first, then with a video on the techniques applied to a horse that is rearing.
It is important to address the reasons a horse rears, before trying to address it.
Horses usually rear when they want to go somewhere and are prevented from going, or if they don't want to go forward and are forced. They can also be confused by having contact introduced too early in their training, for example a rider driving them forward while pulling back on two reins, the horse will be getting confusing signals and may actually think a rear is the right answer.
A horse should be able to walk, trot and canter on a loose rein and understand forward properly before contact is introduced.