Judge Gerald Lebovits has an excellent article on email for lawyers in the New York State Bar Association Journal. It's comprehensive. My favorite tip is this:
Fill in the address box only when you’re ready to send. The ease of sending out mass e-mail, purposely or inadvertently, means that you must take care when addressing your message. To avoid sending an e-mail before you’re ready, write your entire e-mail, do all your edits, and proofread before you fill in the address box.
This is a nice example of poka yoke. The process helps the writer to stop and think, minimizing potential errors when composing the body of the message. It also clears the mind before selecting the recipient, making it more likely to be the correct choice between sometimes similar contacts. And for lawyers, this can be a critical choice, to wit.
In addition, I frequently add my signature to an email only after completely editing it, especially important correspondence. It forces me to be sure I really want to associate my name with what I wrote.
D. Mark Jackson