My colleague recently showed me http://www.fitsw.com/ to help me manage my training business. I like it because it’s FREE and has a lot of features to help with managing client progress (cool graphs!), and a lot of features to manage workouts, diet etc. I can even integrate workout demo’s into the workouts for my clients.
Another one I had used previously that I liked is http://www.brothersoft.com/gym-trainer-166395.html but it is too expensive to use with my own business (my old gym used to pay for it).