There are sometimes very specific, difficult-to-answer questions which are left unanswered. I just had an idea that might solve this problem:
How about a growing reward for questions without answers and without down votes, owned by an active user (who is likely to look at his old questions). Then other users would be more motivated to browse through old, unanswered questions. And the questioner himself would be more motivated to write how he solved his question after he gave up waiting for an answer.