They will happen if you've been in this industry for more than 30 seconds and, let me tell you, it can hurt. How far your business will go is reflected in how you handle the cancellations that come your way.
The best method that I've ever hear for getting over the cancellations is to use that time slot for your business at home. For example, if you had a Tuesday 7:00pm show that canceled, you would use that whole time slot to do other business related things like bookings, follow ups, and some administrative activities. If you use that time for income producing activities (like bookings or follow up) you will not be moving backwards due to that one cancellation. If you are warned far enough in advance, you can try to put another booking in that time slot. I might even offer a very special incentive for anyone who books in that spot, so you know you can get it secured.
The most important thing is to try not to let your emotions get away from you; 99.9% of the time it's nothing personal, and sometimes it can even be rescheduled for a better date. Don't forget that this business is all a numbers game and, if you plan right, all of your shows will even out with the industry percentages. Ask anyone that's been doing this for a while, you will have lots of cancellations during the time that you running your business, sometimes they even be yours. Stuff happens. Stand up. Dust yourself off, and keep going!