There are two basic schools of thought on moral praise:
Be generous: Praise people generously, telling them that lots of things they do are moral, even marginal acts that seem questionable. This gets people into the habit of thinking they are especially moral, and makes them more receptive to later requests to act moral. Better to have people pretend to be moral than not to care at all about morality.
Be stingy: Praise people sparingly, and only for acts that seem clearly and strongly moral. Since people want to be moral, they will try harder to meet your higher standards, which will induce more moral behavior overall. It will also better ensure than their moral contributions are real, and not just what folks like to think are moral.
Recent studies seem to favor the stingy school:
It seems that we have a good/bad balance sheet in our heads that we’re probably not even aware of. For many people, doing good makes it easier — and often more likely — to do bad. It works in reverse, too: Do bad, then do good. …
Voters given an opportunity to endorse Barack Obama for president were more likely to later favor white people for job openings. … people who bought green products were more likely to cheat and steal than those who bought conventional products. … After getting high-efficiency washers, consumers increased clothes washing by nearly 6 percent. Other studies show that people leave energy-efficient lights on longer. … Choose between buying a vacuum cleaner or designer jeans. Participants who were asked to imagine having committed a virtuous act before shopping were significantly more likely to choose jeans.