Another great source for solutions to underachievement is Dr. Del Siegle at U Conn. His workshops are wonderful, and he kindly provides a lot of his research and presentation materials online. For a longer read, check out his work on self efficacy.
A key point that they both make is that, "no one wants to be an underachiever". Accepting that idea is a very important step in the search for improving the lives of the people that we see as underachieving.