Write a detailed biography for each of your characters. It's essential to get to know the inhabitants of your created world. This includes creating back stories -- how was the character raised, is he single or married, what kind of work does he do, what are some of the moments that have defined his life and impact how he makes decisions? Incorporate some of these events into your story.
Make two columns on a piece of paper. On one side, list all the things you love about yourself. On the other, make a list of all the things you dislike about yourself. Give each of your characters at least one trait from the "things you love about yourself" column. This will ensure that each character has at least one virtuous trait from which to act.
Demonstrate the virtues of your characters through action, rather than simply telling the reader that a character is virtuous. Examples of this may include standing up against the group for what's right, defending the weak or sacrificing one's own interests for another's benefit.
Challenge the virtue of the character. Put the character's noble qualities to a test from which he can emerge even more virtuous than before. A moral dilemma, where the character has to choose between the lesser of two evils, is an example of such a test. Virtue is less impressive if it's not tested in some dramatic way.