E-mail etiquette and best practices, aka, how not to be a complete fracking n00b

I was about to educate a co-worker on some of the better practices to follow while communicating on the Internet, and being the generous chap that I am, decided to share these invaluable nuggets of my infinite wisdom with you lot. So, here goes:

1. Format: Never send e-mails in HTML format. Always use plain-text only.
2. Subject: Always use a descriptive subject line. Subject lines such as "Hi" and "Please help" will get your behind flamed.
3. Remember the medium: Always keep in mind that you are communicating via an electronic written medium. Sarcasm, wit and obscure attempts at jocularity do not carry very well. Use smileys and other visual cues to project the right tone and mood.
4. Grammar and Style: For the sake of all that is holy, avoid SMS speak in e-mails. Advertise the fact that you passed fifth grade by using proper grammar, capitalising and terminating your sentences accordingly, structuring your e-mail using paragraphs and so on. Errors such as using "would of" instead of "would have" will result in the sender being drawn and quartered.
5. Attachments: Try to avoid them in general and link to a file on the Internet instead. If you do have to send attachments, use a little common sense when it comes to choosing the format. Try to avoid crap like .doc, .xls etc. which are usually harbingers of doom (read: viruses), unless absolutely necessary. Regardless of the attachment, consider compressing it using RAR or ZIP or if you are sending a screenshot, crop and optimise the image prior to sending.
6. Snipping: Trim away all the excess fat from the e-mail. This includes irrelevant conversation, signatures and other junk. Keep the e-mail as concise as possible.
7. Top-posting: Never top-post. Top-posting is where your reply is above the original e-mail.
   Top-posting (bad):

   My name is Mud.

   On 01/02/03, anonymous coward wrote:
   > What is your name?

   Bottom-posting (good):

   On 01/02/03, anonymous coward wrote:
   > What is your name?

   My name is Mud.

   The latter is obviously more legible. This is especially important in mailing lists.

8. Disclaimers, signatures and other junk: Nobody gives a rat's rear-end about your company's disclaimer. Nobody gives a toss about your cute little signature - no, we don't want our days brightened by insipid witticisms. If you'd like to advertise your contact information, try to do it via a link to a contact page rather than appending 10 lines of redundant text to every e-mail.
9. Privacy: When you forward e-mails, respect the original sender's privacy and do not disclose his name and e-mail address. Snip, snip, snip. That said, when you send an e-mail, don't expect it to remain private either :S
10. Forward Spam: Do not forward crappy chain mail with cutesy nonsense to everybody in your address-book. No, your loved ones will not die if you don't do it within 60 seconds.
11. Threading support: Avoid mail clients and web mail services that do not support threading. Threading allows an e-mail and its replies to be grouped together making it easier to follow and organise. This is especially true for mailing lists as a reply that does not respect threading will fragment the conversation.

Well, there you have it. I can come up with more, but they will probably be more of a personal preference or annoyance rather than something I can postulate as being for the greater good of mankind.