"We are really pleased with the experience of working with you and I'm sure we will need you in the future. Again, thank you for a job well-done."
I just wished I had hired you from the beginning...
“Thanks for all your work thus far. More has been done with these forms in the past few weeks than was ever done previously by the other people I hired. I'm happy and impressed with your work and the timely manner that you are getting everything done. I just wished I had hired you from the beginning!”