First off, discuss the issue with your CFI. Tell him that you're tired of cancelled lessons and ask him if he forsees a problem meeting your scheduling needs in the future. It might just be that he had a spat of legitimate reasons for cancelling. If he promises it will get better give him one or two more chances... if it doesn't then don't hesitate to bail out on him.
If you're at a larger flight school, ask to talk with the chief instructor or the manager. Calmly explain why you don't feel that your CFI is meeting your needs and expectations. If they don't offer a solution, suggest the possibility of working with a different CFI.
Take it one step at a time and escalate the issue as you see fit (warn your current CFI, ask to fly with another, leave the flight school). Remember, the flight school charges you for cancellations, it's not fair if you clear your schedule and then have the flight school cancel for unjustified reasons either...
~J