I don't know how many e-mail clients support this, but Outlook 2007 at least offers an option in Client-Side Rules to delay sending of e-mails for a set # of minutes, along with some exceptions such as a high priority or certain To lists. I'm using this now to help catch me even if I do hit send. I still practice a lot of what has been said above, especially if I'm pretty emotional about something. However, this has also helped me stop e-mails from going out about topics that were answered in a group e-mail I just received or sometimes to add/remove people from the list.

It's been useful to delay the speed of e-mail just a little bit in case I have some second/third thought about what was in the e-mail.