I think Barbara's idea of holding off on actually hitting “send” is a terrific one. It's also a really good habit to develop not just because it could save the recipient some angst/rolling of the eyes, but it also helps the sender feel an item's been cleared off the “list,” and is ready to go come business hours – so a time saver, in many respects. Because I do know the feeling of wanting to capture the idea(s) the minute they come to you – which is, I think, the reason a lot of people send the instantaneous emails at Godawful hours.

I also think this is a tactic that a lot of us need to use more on the response end, particularly those of us with clients. Not because we don't want to talk to them, but because if we respond immediately, we're conditioning them to feel we will ALWAYS respond immediately, regardless of whether it's warranted or not… and that can lead to some discomfort down the line. I mean, who wants to be Pavlov's dog?

The other thing I've learned to do is, when an email is urgent, not use the little exclamation point (as you point out, those are over-used), but actually put “URGENT,” “TIME-SENSITIVE” etc. in the subject line of the email. That, at least in my experience, really works.

Now go take a break.