I have to repeat the question, is everyone friends here? Or are you playing with strangers?
One thing I've noticed is if someone is having a bad day, typically they'll bring their stress to the game. A big key thing I do with my players before we start is sit down for 30 minutes and catch up with my players on how their week's been. A lot of people will get the grief of there crap job, relationship issues, life issues out in the open during that time, and it generally makes for a better game, as people are aware that one person is stressed and they at least be a bit mindful towards them during the game.
Another big thing I've learned is it's good to be friends with people outside of the game. Go watch movies, go out to dinner, etc, but be friends with each other outside of the game. Simply because it's easier to ask people to be more mindful during the game since to a stranger asking them to behave sounds more like an insult.
Anyway, hope that helps.