It helps to keep conversations factual and prevents emotional reactions. This includes refraining to swear.
Saying "Hello!" when you join and saying "Bye Bye" when you leave is also part of being polite. Some "Hellos" may continue with an additional question and conversation which helps getting to know each other. When someone is asking you something you should answer.
Be open minded.
Don't make it difficult for youself. Accept other people's critisicm as their opinion, don't judge people instantly and be open for new developments.
Be self-confident and honest.
You think the idea of someone, who is much longer in the clan is complete rubbish? Tell him. (In a polite way of course) Always position yourself where you want to be positioned and feel free to bring counter-arguments at any time.
Do not map wreck.
Try to ask for permission before you start editing someone else's map.
Do not cheat on fighting servers. It will make you and your clan infamous. Coop mode, and with everyone's permission is fine.