Motivation comes from inside. No matter how great you are as a personal trainer, your client’s adherence to his/her program comes from his/her commitment to having a better life. You can only influence it by the way that you live your life. Run your marathons, ride your tri’s.
Ask your client what is really important in his/her life. The ultimate decision comes downs to his/her decision.
I tell my clients that they won’t always be motivated to workout. That it is not a problem. When they are lacking motivation I tell them to “fake it”. It has been shown that “faking” motivation or interest or any target attitude can produce nearly equal results in productivity, retention, and satisfaction. I often hear many of my clients using my phrase “Fake it till you make it” when encouraging other gym patrons as well.