Emails are great...until you send the wrong one to the wrong person. Don't you wish there was an undo button AFTER you hit "send"?! Apparently, if you use Gmail, there is.
In the "settings, labs" tab, enable the "undo send" feature. It delays your email from actually being sent for 30 seconds after you hit the send button, giving you a grace period to realize what you've done and undo it.
If you don't use Gmail and your email server doesn't have an undo feature, try this: don't type in the recipient's email address until you're done composing the email. That way, you can't accidentally send anything to someone.