I have too many email accounts: a few with my ISP, one .Mac, and a few with my web hosting service. Some are used for web business, others for support email, and others for personal email. When I create a New email, Mail picks up the account for the currently highlighted message in my inbox. More often than not this is the wrong account. Worse, I only think to check it after sending.
By adding a reminder line ("Double Check The Account!") at the top of my Signature, any New, Reply, or Forwarded email will have a built-in reminder for me to make sure the email is being sent from the correct account. After I set the account, I delete the reminder line from the email and type away.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040226092709515