Read the message board for a while before participating (or this thread)
It's best to browse a board for a while -- just reading -- before you post your first message. Read a couple week's worth of messages the first time you visit a board, and then read new messages for another couple of days. You'll see what topics are hot and which are cold, you'll get to know some of the key participants, and you'll become familiar with the demeanor of the board. Each one is different.
Communicate clearly
Write clearly when you compose a message. Review your message carefully before clicking Send; typos are confusing. Remember, if your message can be misunderstood, it will be.
Refrain from using ALL CAPS (hyper's addition: and lots of punctuation)
Typing in all capital letters (and using lots of punctuation)is frowned upon; it's the equivalent of screaming at everyone. It also makes your message more difficult to read.
Don't post meaningless messages
Posting messages that say nothing more than "Me too" is a quick way to irritate other members. If you agree with a posting, either enjoy your consent by yourself or reply with an informative comment about why you agree. Imagine how dull a board would get if it contained nothing but hundreds of "me too's."
Consider sending a reply by e-mail
Some replies are better sent as e-mail, rather than posted to a board. Personal messages and especially critical comments are more appropriate when sent directly and privately. There's an E-mail to Author option on every Post Response form; don't forget this option.
