There is more to life than any one sport. Encourage your child to participate in other activities. Respect the coach's decisions and abilities. Don't force your child to decide whether to listen to you or the coach.
Stress fair and sensible play. Nobody needs to get hurt because someone is not playing by the rules. Stress the long-term importance of practice over the short-term excitement of games. Stress the importance of the team. Never lose sight of the fact that you are a role model.
Winning probably means more to you than it does to your child. Don't yell if your child makes a mistake. Point out what they do well. Positive reinforcement goes a long way. Take time to learn the rules of the game. That way, if you insist on shouting at the ref, at least you'll know what you are talking about.
Remember that coaches and league administrators are volunteers. A "thank you" every once in a while is often reward enough.