The unanswered questions section on SuperUser shows questions with "no upvoted answers", but often has questions with three or more answers and 50+ views. I'd guess that many of those viewers may be viewing with a specific answer to provide but see that it has already been offered.
How about putting up a banner to suggest that people upvote one of the answers?
The banner could be shown on basis of criteria such as:
- question open > 5 days - don't show on questions which are too new
- number of answers > 2 - more answers means more are likely to agree with one
- number of views for this question > 25 - more views means more likelihood that one is a good one (in somebody's point of view)
- viewer's ratio of upvotes to answers given > 0.5 - Don't show the banner to those who are good about upvoting other people's answers and not just answering questions to boost their own rep.
The numbers are off the cuff and not based on any analysis of data.