Count your blessings, I'd say :-) Many have trouble ranking well in the three major search engines simultaneously. Having said that, I think ranking #13 is likely something you can build on. Since you're ranking well in Google and Yahoo, changing things on-page for the sake of ranking better on Bing does not sound like a good idea to me, so I would "simply" focus on acquiring more quality links to the site - in fact, that should help your rankings on all three.
I get the feeling Bing is putting a relatively large amount of weight on traffic data, so perhaps seeking visitors from other (quality) sources might help you rank better as well. I should stress that this is only a casual observation of mine; I can't back it up whatsoever.