"Just wanted to drop you a line to thank you for your assistance in placing me. What I particularly liked about the process was your thoroughness and reliability."
"Just to say thank you so much for all your help in finding me a new role, you have been fantastic. I've dealt with many agencies whilst looking for a new role."
"I just wanted to drop you a line to say a very big "Thank You" for your help in my recent success with my job search."