cookies,
I like cookies
Seriously, not really sure what my parents did to motivate me. It's something that everyone struggles with from time to time. Heck right now I'm looking for the motivation to go to bed.
Ye ole pep talk about goals and objectives. See what they want and show them how getting them to do what you want them to, will help achieve said goals. I would think (though I don't have any kids, far from it) that would be the best way.
If not,
cookies
I like cookies