1. Keep up the good work!
2. That was a nice try. (say this after someone made a good effort but failed)
3. That's a real improvement.
4. You're on the right track.
5. You've almost got it.
6. You're doing great.
7. Don't give up! / Hang in there!
(when someone is currently having difficulties)
8. You can do it!
9. Give it your best shot. (best shot = best try, best effort)
10. Nice job! / You did great!
(after the person has done something good)