Another useful trick I've found is to automatically delay my email responses. Rather than replying right away, I purposely let email sit for 48 hours or so. If people get an instant response, they'll send more useless emails.
Of course, this doesn't apply in every case. If I get something that I know is important, and the person isn't wasting my time, I'll respond instantly.
Of course, this doesn't apply in every case. If I get something that I know is important, and the person isn't wasting my time, I'll respond instantly.