"I think you can do better next time "

You have to work hard and cheerfully!

Onni is correct as to your desire to want to see someone work harder. It all depends on the particular circumstances and your relationship to the person with whom you are speaking as to how you might say it. Onni's answer is one that shows respect and kindness. "Get you rear in gear and get it done," not quite so nice but is how some might say the same thing.

Hmmm...to work harder and cheer up at the same time?