To everyone's amazement these haven't been posted anywhere before- we generally just say that our rules are a mesh of our server rules and typical forum etiquette. If you know both of those things, then you should be just fine. However, so that everyone is clear, here are some guidelines:
Be respectful
No chatspeak
No trolling
No derogatory language
No advertising
No overtly sexual content
English only
Necroposting: If new information comes to light in an old topic and it's useful to post, then this is okay. However, there's no reason to post a casual remark, such as a greeting or agreement, in a topic that's more than a couple months old.
Double posting: If there is a reason people need to see the topic again after a long period of time after your old post, this is okay. For instance, suppose you were the last post in an Art thread but made a new drawing several days later and want to post it- fine. However if you're making several posts in a row all at once, there's no reason those shouldn't be kept to a single reply.
Signature length: Our general rule is two-trainer card heights (not counting your trainer card). If it's longer than that, consider a Scroll or Spoiler tag.
Quality of replies: Make sure each reply is on topic and has a purpose beyond simply saying you agree with something or think it's cool. If you must post those sorts of things, at least elaborate on why you agree, etc. You earn $R for each post, so make sure your post is worth the $R.
If there are any questions on exactly what these mean, you are, of course, free to ask for clarification.