I don't get annoyed by cancellations, but if you do, please don't do the following:
Please, do not tell me repeatedly about how "just as soon as my schedule opens up" or "just as soon as I have the funds" that you'll reschedule. Particularly, if you review 5 or 6 new visits during that same time frame. I'm not your girlfriend, you really don't have to make any excuses.