Rule 0, the most important rule: Have fun! This is a leisure activity after all!
Rule 1, can supersede Rule 0 if necessary: BE RESPECTFUL. You don't have the right to take away someone else's fun.
Rule 2: Listen to the General. Seriously, he is responsible for trying to keep everything cohesive. He may not be perfect, but he is trying to provide a larger story that everyone can be a part of.
Rule 3: Don't post other character's actions. This is what Joint Posts are for. Each player is responsible for their character's actions.
Rule 4: Pay attention to NPCs. When a player creates an NPC, they generally create a personality to go with it. Try to get into that personality if you use them (or better yet, include the owner in a JP).
Rule 5: Violence and Sexuality. Keep it respectful. Don't go overboard with it. I don't mind if it is tasteful or advances a storyline, but it should not be the entire contents of every post you make.
Rule 6: Refer to Rule 0. Seriously. Have fun; this is a hobby, not a job.