As a good manager, you should encourage great workers and bad workers. Good directs need encouraging to keep them working hard. Bad employees need encouragement to work harder. Here are some sentences for both types.

"Hey James, I think you're doing a great job and it is not going unnoticed."
"You're doing great. Keep up the good work."
"I told my manager about your performance. He was quite impressed. Keep up the good work."

Directs having difficulty

"I had a hard time on my first project too. Don't let it get you down. I'm sure you will do better next time."

"It wasn't as bad as you think. I also saw definite improvements so you shouldn't give up."

"Your work has been pretty good, but I really think you can do better. You have potential and I hope you start trying a little harder."