Advertisements, Be Gone!
Quite simply, a forum is not an appropriate place to advertise your own commercial services or events. Assailing the forum with unsolicited ads like contact information for your new housecleaning business, information about the poetry chapbook that you've written, or news of a sale at a record store that you own will quickly make you the most unpopular kid on the block. When you encounter other forum members violating this tenet, let the moderator know. Don't bother chastising the poster.

On the Down-Low
Revealing your own personal information (i.e. address, phone number, photos) on the Internet is unwise. It's like an open invitation to telemarketers, identity thieves, stalkers, and other undesirables to come flocking to your door. Revealing the identity of another person is equally dangerous-and a blatant violation of that person's privacy. Don't do it.

Be Honest
Tsk-tsk: Masquerading as another person, whether a fellow forum member or someone else in society, is clearly not okay. Sure, the web allows you to experiment with your persona a little, but it doesn't grant you permission to lie outright. It's also uncool to post images or words-copyrighted or not--that someone else wrote and claim them as your own.