At least for high school motivation, my father explained to me that, in order to graduate valedictorian, I would have to work extra hard in school and avoid getting B's because grades actually counted once you got into 9th grade.
So, if you have a hypercompetitive, win at all costs child, who is currently coasting, that is one approach.
However, having gone through this experience, I recommend avoiding it since it doesn't tend to develop intrinsic motivation and does tend to develop anti-social behavior.
