"I just wanted to drop you a quick note to thank you.  When we started working together last year, I had no idea how far I could go with getting in shape.  I am totally different now: my eating is much improved, I am running 12 to 15 miles per week, boxing and lifting weights.  I am in the best shape of my life.  And I still do a bunch of the exercises and routines that you showed me.  Thanks for helping to inspire me and for giving me some concrete ways to take my workouts with me on the road."