Several threads caused me to think of this. Ten years on the boards, and I assure you I have violated every one of these rules at one time or another...to my ultimate regret.
1. Don't be judgmental. Remember that you, too, are spending time contributing to a SHMB.
2. Don't preach. The people you are trying to convert aren't going to pay attention to you anyway.
3. I can't tell you the number of times I have finished typing a witty, scathing, memorable-for-all-eternity post, only to pause for a minute before moving on to something else without hitting the "Submit" button. Not once have I regretted that decision.
4. Treat everyone as an adult, even if they aren't acting like one.
5. Don't take the Hobby seriously, but take your personal security and privacy VERY seriously.
Merry Christmas everyone!