Be mature. This means be helpful, be polite, and if you have an issue with another member don't bring it into guildchat.
Be helpful. The whole guild benefits if everyone can get levelled/geared/trained/etc. Take some time to help lowbies with tough quests, or a fellow 50 get through a few gear runs.
Be respectful. In game or in vent, avoid topics that might ruffle feathers. If it offends someone, it is offensive.
Don't be afraid to speak up! Jump into a conversation (it's the best way to get to know people better). If you need something (crafting mats, gear, help, etc), just ask! You'd be surprised how many bored 50's there are.