I recently saw this well-done brief article on CNN.com about the worst business-related e-mail offenses everyone seems to encounter. It wasn't aimed at the law, but much of it applies to us.
Here's an abridged version:
DON'T USE ALL CAPS.
Don't use Comic Sans or similarly dumb fonts.
Get the other person's name right.
Don't use emoticons.
Avoid overly informal language.
Keep it short, get to the point fast.
Subject lines should contain a subject, not "Hi". (On a similar vein, I used to have opposing counsel who started typing the entire message in the subject line, and just kept going when it bounced over to the message body section. Really. Damned. Annoying.)
Do not send multiple e-mails in a row, or harass a person for not responding immediately. That's what phones are for.
Clip Art. Don't. Just, don't.
Use the right method. Facebook is not acceptable for business contacts.