Emotions are often hard to convey and easy to misunderstand in written text. Smileys and other indicators can help (but don't assume that attaching a :-) to an insult will make everything OK with the people reading your post.).

DON'T POST IN ALL CAPITALS or in all bold or, even worse, IN ALL BOLD CAPITALS. This is considered to be shouting, and most people don't like to be shouted at, whether in real life or on-line.

"Shooting the messenger" is seldom a good idea. In general, assume that people who take the time to reply to your posts are honestly trying to help. Getting mad at them and "flaming" back is counter-productive if you really want people to help you.

Replies to posts will often be short and to the point simply because the responder has limited time. Don't mistake terseness for rudeness.

Many people who post questions and requests for help may have made very basic mistakes. If you omit the details of everything you thought of and checked before making your post, don't be insulted if someone replies with a very basic suggestion or a link to something that you have already read.

Don't "hijack" existing threads to talk about a topic different from the original poster's topic. Start your own thread instead.

In an ordinary conversation, no one appreciates the person who barges in and insists on changing the topic of dicsussion. And if two groups of people actually insist on trying to simulataneously carry on a discussion on two distinct topics within the same conversation, the result is usually confusing to everyone.