Agree with what others have said, but one of the best things you can do is learn how to recognize a good try, and when a horse is really working for you. Praise, pets, and letting him know you're happy with him when he does well makes his job a little easier.
Other than that, learn to be considerate of his mouth, and how to give a quick release of pressure when he does what you want. That will come with time and experience.
Physically, I like to scratch their withers, chests, where the girth sits, and on their butts if you use a crupper when you untack. Little things like that make a big difference to a horse that works for his living. Watch their lower lips, you'll see them droop and wiggle when you hit a spot they like :)