Handling Cancellations

We just got a cold blast from the north, and my schedule has suffered. No, there was no snow, but a client called to cancel and another was running 15 minutes late. Which brings me to the question: do you charge for cancellations? Usually I don't charge for cancellations made in advance of the appointment. They don't happen often and usually there's a good excuse (isn't there always?). When I get to a client's office, I start my clock at the agreed upon hour. If they are late, they pay for the time I waited. If they don't show within 15 minutes, I leave and charge them a half hour for my time. How do you handle cancellations?