This multimedia lesson does an excellent job of explaining Daniel Pink's research on motivation. This video and the suggested further reading are great to follow up with an application activity where students identify what motivates them personally.
This article gives great research-backed examples of how motivation is essential to our success. I particularly like the section on effort vs. goals because it explains the importance of working on a focused goal rather than just working hard in general.
This article touches on goals as well as motivation, and then gives examples of how high achieving athletes and public figures stay motivated. I love the videos because students can see what it really takes to achieve big aspirations.