I have many clients who I worked with for years. Their motivation is and was to improve their health and maintain an active lifestyle as they are getting older.
It seems that I am fortunate to get clients when they are truly ready to commit to an active lifestyle.
As others have stated, it has to be an intrinsic desire. Outside motivators really do not work very well but I found one notable exception: GRANDCHILDREN! I often found clients to be very motivated to be there for the little ones and to maintain their health for their sakes.