It's easy to fire off an email with barely any effort at all. And that's exactly how much effort goes into most emails: none. Ole Eichhorn's Tyranny of Email offers a succinct set of guidelines to avoid thoughtless email abuse:

1. Never criticize anyone in email. Avoid technical debates. Use face-to-face meetings or phone calls instead.
2. Be judicious in who you send email to, and who you copy on emails.
3. Observing some formality is important.
4. Don't hesitate to review and revise important emails.
5. Remember that email is a public and permanent record.

Ole published another article, Tyranny Revisited, with some commentary on the response to his original article.

43 folders recently published a great article on writing sensible email messages:

1. Understand why you're writing: what's the goal?
2. Assume no one will read more than the first two sentences of your email.
3. Write a great subject line.
4. Fit it on one screen with no scrolling.
5. Ask for what you want.

Similar rules apply to instant messaging, telephone, and even face-to-face conversations. The key difference is the amount of effort required for each communication method: the easier it is, the more you need to consider before doing it.

Of course, the real art is knowing when to escalate from IM to email, from email to face-to-face, and when to drop the ultimate communication A-bomb: calling a meeting.