From the PvP side of the house:

1. No quitting. I've turned too many "losing" games around, and I'm not about to roll over for anyone. It's important to know when you're beaten, but never quit.
2. You miss 100% of the shots you don't take. This pairs well with #1.
3. Be humble, even in badassery. I strive to stand out as an awesome player, but I haven't forgotten when I was just a nobody getting no heals and no support.
4. Support your team. Friends, rivals, even bitter enemies; if we're on the same team, I have your back.
5. Help where you can, teach when you see it's needed, and allow room for failure when failure will be a better teacher.
6. In the event you make a mistake, own it and learn from it. Not everything I do is a great idea that works either.
7. Be a good sport, but if you earn it, go ahead and celebrate.
8. Salt, hurt feelings, and bad attitudes: leave it on the field or in the arena. No sense burning bridges when everything else in game calls for cooperation.

And on a very serious note in general: Reach out to people and pay close attention to changes in their behavior. Simply being there for some people when they needed it has prevented much worse things from happening this year.