Potential

It's actually counter productive to tell someone that they have a lot of potential and can achieve great things if they tried. This only helps them build up an ego shield that they can use as a layer of protection.

You've just got to be completely honest and tell them how stupid and disappointing they are for not using any of their potential. You really need to annihilate that ridiculous undeserved ego for them to be able to truly tap into their potential.