According to the article written by Bridget Murray, (see link above for full article), verbal praise is considered the best motivator of all.  Although there are some negatives about verbal praise written in the article, the overall feel is a good one.

 

The following is an excerpt from the article:

"A better strategy is praising children's efforts to learn rather than their success at learning," Dweck has found in recent studies. "Praising children's effort encourages them to work harder and seek new challenges," she says. "In one study, for example, fifth-graders were most likely to attempt tough math problems when researchers praised them for working hard and trying new strategies."