The greatest lesson that any Rider can ever learn is to Keep Your Heels Down. Practice--Practice--Practice. You really just have to keep your mind on your heels until it becomes your physical nature to have them so dropped that it is excessive.
It helps if you can have someone lunge you on a horse, so that you do not have to hold the reins, or steer. Practice riding without reins. At the walk, then trot, then canter, then trot over poles on ground, canter over poles on ground, and eventually trot--then canter over small jumps (try to not go over 2 feet 6 inch fences) on a lunge line or without using your reins. Even in Western Riding, you must have your lower body balance without use of your reins, and to pop over small fences will not hurt your ability but increase your seat for Western Riding. On the trail, you will have to sometimes pop over small logs on the trail, and to do so and not pop your horse in the mouth, or to loose your seat, these exercises will only help you to do that.
While you are riding, put your hands on your waist; then on your shoulders, then head, then strait up in the air over head, then strait out from the shoulder, then you can become very fancy, and touch opposite hip side or knee side, and eventually the toe of the opposite leg.
To maintain position doing these exercies, YOU MUST KEEP YOUR HEELS DOWN, or you will pop out of the saddle and always be fighting your position and balance.
