Practice Pointer of the Week
Why didn't I think of this? Why didn't somebody tell me this before? Teach yourself to compile your emails from the bottom up. First, if there's an attachment, attach it first. That way you avoid having to send the "Oops, this time with attachment" follow up. Then compose your email before you address it. That way you avoid firing off an incomplete email. No tip yet for avoiding mis-addressing emails other than never use 'reply to all' and never trust auto-fill.
Thanks for the tip to Matt Holmann of LexThink.