1. Under normal circumstances, never use capitals! It's the equivalent of shouting. Also, be careful with the exclamation mark.

  2. When replying to an e-mail, remove the headers and answer below the old text. Example

  3. By far not everyone can read HTML e-mails. So when you e-mail someone you don't know, send it in plain text.

  4. When reacting in a forum or newsgroup, remove the old message, unless you're replying to a specific part of the message. The old message is always easily readable again. 

  5. Don't reply in a forum or newsgroup when you don't have anything meaningful to say. Especially the newsgroups are very crowded, so useless messages won't be appreciated.

Follow these rules, and you'll be virtually dignified.