For those of you who have never posted in an RP forum, here are some guidelines... These are important! They make the experience enjoyable for everyone if you follow them.
Background is not always needed, but helps so that you don't have to explain everything you do. Tip: Post background info in the character databases.
It is important that you know and prepare your character's flaws as well as weaknesses. If you don't develop weaknesses, your character will dominate, which is unfair to other RPers.
Never, EVER control someone else's character! This includes speaking. Why? Because you might be doing or saying something that character wouldn't do. Tip: If you want to prod someone to say or do something, leave your post open-ended, and they'll pick it up on it.
A. Keep it moving
Remember to keep the story moving. Out-of-character notes can be added in, but make sure that something is happening, or someone may end up stuck somewhere.
When you're RPing, remember that every story has to have some kind of plot. RPs generally have more than one plot, but each one has to end somewhere. If you get to the point where the storyline isn't solid anymore, get out of there. There is nothing wrong with continuing the adventure in another thread -- it not only allows room for a new plot, but it also makes it much easier for late joiners to follow along. Note: When's the last time you joined a thread late and had to dig through 4 pages of story? It sucks. There is no shame in creating a new thread for simplicity's sake.
That's all I can come up with. If anybody wants to add on, please do so.