I do know of one method that’s been around for a while. Good old paper and pencil. I like it. That and using Microsoft Excel. These are great tools to create a scheule of your availability and to keep track of the clients your training and when. I advocate the most labor intensive method. I know that sounds odd, but this way I am sure to put the effort into it that aspect of the job that it deserves. It gets easier with practice. I think it’s the most reliable way to keep track of any schedule.
You can go and tryout mindbody. http://www.mindbodyonline.com/
Also a platform that’s being used very successfully (and very cost-effectively) by numerous trainers, teams, and gyms is called LogsItAll. It’s easy to use, flexible, scalable, customizable, and the company’s founder, Bill Patton, is an athlete, website is, www.LogsItAll.com, under the “For Trainers” tab
MYZONE is another affordable way to engage with and retain your members/clients. www.myzone.org is the website
Sorry I am not sure how long either of these have been around but you can ask that question.