The most obvious rule is, don't try to destroy things. I know that the kind of people who even bother to read this won't normally be prone to actively destroy a MUX, but I'll say this anyway. The most common excuse after someone has done something damaging to the MUX is to say "But you're a wizard, I thought you could fix it." In many cases, restoring a database from object-spamming or other attacks is a difficult process, not something a wizard likes to do in his free time.
As well, don't do things "just because you can"; remember that other players may be faced with the results of your actions. For example, if players leave their objects unlocked, it is possible to go and pick them up, but there is no reason to do this except to make people hate you. "It was just a joke" is not an excuse.
Now, having covered all the extremely twinkish stuff, we can proceed to the more subtle things.
Speak good English. Over the 'net, people can't judge you by your age or appearance, they can only judge by how you talk. Using B1FF5P33K is right out. Simply being lazy at typing, like misspelling words or misplacing spaces, also makes you seem less credible. Capitalization seems like a picky detail on MUX, but it's surprising how it can affect the tone of a sentence.
Remember that a MUX is not a chat room. Asking "age/sex" is a very good reason to be mocked, since the sex shows up on the +who list, and age isn't supposed to matter on a MUX, and it's just plain stupid. For the same reason, don't put numbers in your name; it is considered trying to imitate the player who has that name, and if nobody has that name to begin with, it's again just plain stupid.
On the topic of names, don't take the name of a major character from the theme just because it's available. There's probably a reason nobody has taken it. (On SluggyMUX, using names of major characters from Sluggy Freelance is not allowed.)
Above all, use common sense. This guide can't possibly tell you everything you should avoid doing. You'll just have to use your brain.