Always make sure you have a cancellation policy for any workshops, classes, or events you may offer and that you spell it out clearly for your customers. You don't have to be mean about it, nice words will do.
I used to worry if I put my policy out there, then people would be reluctant to commit, but I've found that isn't true. People respect me for kindly and gently letting them know what to expect and it alleviates disappointment on my part when I know I can count on something. .