Seth Godin’s got a nice little “Before you send that email” checklist at:
http://sethgodin.typepad.com/seths_blog/2008/06/email-checklist.html
My personal favorite is #28 (check snopes before forwarding emails about viruses or charity efforts).
On that same note, we’ve posted an email aptitude test that we propose all new email users should be required to take: