Use explicit statements, graphics, a sound effect, or similar indicator to reward a job well done.
Praise is a form of feedback that fall into the category of rewards. The opposite of praise if blame or derogation. Where blame or derogation are tools for negative feedback, praise is a positive feedback form.
Consider every moment in the experience you have created for your users and ask yourself the following questions for every one of them in order to figure out if praise is right for just that moment:
Praise is interpreted simply by users: the system has judged you, and it approves.
Guide your users toward your preferred behavior by praising it when it is conducted.
1 Jesse Schell, “The Art of Game Design – A Book of Lenses”, Carnegie Mellon University