One thing more than all others: Team is more important than the individuals are.
As soon as kids (or adults) forget that, as soon as how your kid is (or more than likely is not
doing) becomes more important to him (or to you as a parent) than the game itself, you have two choices: lay down the law about how your kid is part of the team and not greater than or more important than the team, and make it stick, or get him out of team
The understanding of how each player supports the team and the team supports each player is sometimes a matter of maturity, sometimes a matter of character, but most often a matter of both. When a kid "decides" -- intentionally or unintentionally -- that how he is doing is more important than how the team is doing, the fundamental purity of the game is corrupted. That's a hard lesson to teach and a hard lesson to learn.