http://www.FitSW.com really has helped improve my personal training sessions. My client’s love being able to track their weights and exercises and it makes things a lot easier than using the yellow cards my gym used to use. It is a quick signup, completely free (I tried a few on the list but then they limit features and charge a lot to use the pro version). FitSW.com provides online tools like workout tracking & organizing, goal/body measurement progress tracking (in very nice easy to read graphs), diet plan creation, allows you to provide demo videos for your clients and includes a scheduling system. Its made my job much easier and you can’t beat the fact that it is Free.
The answers to these past questions might help.