Take a light hold of the reins. Wait until your horse responds by dropping his head and relaxing his jaw, then start lightly bumping him with your feet. If the horse starts to resist or move forward, stop bumping until he relaxes again. Reward the horse's response to your light aids, and in particular, praise him when he makes even a minor move toward your goal. In time, the horse will rock back and start to step backward. When the horse steps back, immediately release all pressure on the reins and stop bumping with your legs (reward). Each time you ask your horse to back, he will get more responsive. Never try to pull the horse backward; just hold and wait for movement, and then respond quickly to his positive response.