Spend time to observe horses in herd. Their behavior will tell you more than any book or video, if only you are open to this information.
Spend time to get to know your horse in the smallest details - you should recognize his every movement, every breath he takes, how he displays his every emotion, etc. That might save his life one day, when you notice something outside of the norm.
Spend as much time in just being with your horse as in riding or otherwise working with him.
Be ready to change yourself in order to change something in your horse.
Be ready to put your horses' needs above your ambitions.
I have come a long way, to surrender my shadow to the shadow of my horse.