It is better, according to a set of University of Pennsylvania researchers, to praise a kid for working hard than to praise him/her for being smart:
If you praise the kid for being smart, he/she will be more reluctant to undertake challenges; the kid is more likely to test limits if they are praised for their effort. When I was in school, the kids who got good grades without much effort were admired; the ones who worked hard were put down as "slogs". Yet, in life, the successful people are those who can pick themselves up after a disaster and keep going. Being risk-averse gets you nowhere.