It seems like a lot of roleplaying games have no problems with players playing antisocial characters who can't really be part of the team. I've been part of groups (in D&D) with both a noble defender of the law, and a brutal tyrant whose goal is to gain power at all costs. The characters don't have much incentive to work together, and in fact have plenty of reason to hate each other. That the group doesn't break down into PC-vs-PC combat is only due to the players being more team-oriented than their characters.
It seems to me like you could add a few extra house rules to promote good team-oriented behavior, but I don't know what they would be.
So what kinds of rules should a game have to promote team-oriented behavior?
Follow-up: I'm actually looking for rules, not gm-techniques. Our group is interested in experimenting a bit with the rules of the game, and we're wondering what kinds of rules would promote a good team. These might be rules about gameplay, or they might be rules about character creation.