The general rule in play-by-post is the same as it is in face to face realtime gaming: add to everyone’s fun, don’t subtract from it.
That said, there are a few guidelines that apply to play-by-post gaming that you can abide by to maximize everyone’s fun.
Rule 0: Lurk First
Every site’s culture and expectations are a little different. Spend some time reading threads and getting to know how a place “feels” before jumping in head first. Read their rules, browse a few threads. Immerse yourself.
Rule 1: Write Right
Not everyone you play with is going to stick with correct grammar and spelling, but pbp is a textual medium. Write to the best of your ability. It’ll make your posts easier to read, and encourage people to interact with you.
Rule 2: Post
Nothing kills a game faster than inactivity. Different games post at different rates, but try to find out what the accepted minimum is, and stick to it. Consistently. If you’re going to be gone, make all good-faith efforts to let the GM know, so nobody is stuck waiting on you forever.
Rule 3: Stop posting so much
At the same time, slower posters can find a flood of new posts intimidating, and if it happens a lot, will discourage them from keeping up with the game. It’s all too easy to get carried away with other fast posters. Try not to post so often that you leave people behind.
Rule 4: Length DOES matter
This can, again, vary from game to game, but find out what length post the GM prefers and stick to it. Some GMs don’t want any one-line posts. Others hate big walls of text. 1-3 paragraphs is a good middle-ground, but if in doubt: Ask.
Rule 5: Add to the game
Your posts should provide characterization, entertain, or advance the plot. Preferably all three. Don’t post just for the sake of posting. Even when keeping to the game’s minimum schedule, your posts should serve some purpose.
Of course, you can only do so much. You can only react to what you’re given. That leads us to rule 6.
Rule 6: Find the right group
As you play on any given site, you’ll find players that mesh well with your own style, and those that don’t. Many of these sites have hundreds or even thousands of users, so there’s really no need to play with people you can’t stand.
Rule 7: If you must bail, bail with style
If a group doesn’t suit you or if you just don’t have the time to play, don’t just vanish. Do the others a solid and let them know you’ve got to withdraw. It’s only polite.
Rule 8: Be cool
Even if you hate each others’ characters in-character, be polite and considerate out-of-character. Don’t take in-character conflict personally. Remember, there are real people on the other side of your internet connection, with real feelings.
There you have it. Simple rules that, if followed, enhance everyone’s enjoyment.