What role does virtue play in establishing or maintaining a liberal polity? What virtues play that role? How can we best foster those virtues?
Ask a group of economists and a group of philosophers the first question and you may see them split down the middle: Economists tell you that you need to get the incentives right, while philosophers insist that it's all about virtue. Incentives are definitely important - but what do we do when the rules, and the incentives to fix them, are wrong? That's where virtue, or something like it, comes in.
I'm convinced the relevant virtues must include humility (we don't know all the answers to complex problems or what's best for others), respect (because we don't know what's best), patience (fostering prudence), vigilance for and tenacity in the face of adversity, and creativity (problem solving). They are built through habit, but must be justified by results and through discussion.
As liberals, nothing should upset us more than when the state forces us to act worse than we are. When laws prohibit us from feeding the homeless, or paying for someone else's parking, we need to stand up for one another. Tenacious insistence on not only our own ability, but the ability of others to act decently - to be free! - emboldens us to stand against further injustice. It builds relationships with those we defend. It teaches us the cooperative skills (bolstered by humility and respect) that are needed to find meaningful solutions to complex societal problems that are otherwise tossed to the government and forgotten.
The virtues that support a liberal society are not what we think of as heroic virtues. They are built on small acts that lie within reach for each and every one of us.