I've been wondering about incentives to prune one's own questions and answers.
For example, in the same way your profile prompts you about "have you considered marking an accepted answer?" (or at least, it used to?) I wonder whether it would be a good idea to add "have you considered deleting this downvoted/zerovoted/notaccepted answer?".
I believe that in the vast majority of cases, stackoverflow's quality would be improved by having less answers.
Another problem is that when you do delete your own answer, you can still see it, and with an ugly colour at that, so there isn't really any aesthetic incentive. How about collapsing deleted answers?
Just thinking out load.
Maybe there could be a "encourage user to prune" flag? (I know lots of people are going to say "that's what downvoting is for" - I'm not so sure. As Jon Skeet has pointed out many times, the 'meaning' of a downvote is completely unknown if unaccompanied by a comment).