Focus on their natural qualities
Every child or teenager has natural talents, skills and passions. I feel that when we help them focus on their passions, we naturally get them extremely motivated.
If they focus on what thrills them, they'll have no problem finding their motivation.
I feel it's essential to give them space and encourage them to go with their natural creative flow rather than leading them in the direction WE think they should go. Take away part of the "controlling" factor in education and I feel what will flourish in their minds is greater than anything we could imagine.
Trust them! That's the best way to build their confidence.