1. Keep the racial slurs, swearing, sexual innuendo, and general tough talking and harassment to yourself and/or to a minimum, we don't need to hear or read it
2. Take defeats and victories with humility and grace, not bitching or rubbing it in people's faces. It's rude and people will not want to play with you.
3. Don't hack the game or over-exploit a glitch, you'll just suck the fun out of the game if you do.
4. Don't act like a tourney tard before, during, and/or after a for-fun match. Gaming is not serious business.
5. If and when you do lose a match against someone, don't make any lame excuses about how the opponent was using a specific weapon or strategy all the time and it's cheap. Most if not all weapons and/or strategies have specific strengths and weaknesses and can be taken down, just need to think it out and pay attention.
In my personal opinion, I think all gamers should follow these guidelines, but it's almost impossible to try and change people to act more mature, but I won't stop trying to persuade people I bump into online and offline to try and make an effort.