My rule: Put the recipient's name in the To: field when you're ready to send, and not a moment before.

If it's a reply, open up a new message and write in that, then paste it into the reply.

That way things don't get sent by accident (wrong shortcut!), and you get a moment to reflect before you hit the send button. Often just having written the email is all you needed to organize your thoughts, and you're better off not sending it. Or sending it to someone you trust for a tone check.