Unreliable bookings

I recently had a firm booking and was then told, after chasing, that the start date was being pushed back. Meanwhile, of course, I've turned down other work and I now worry that the whole project will drop. How does everyone mitigate against this sort of thing? Does anyone charge a retainer when booked or say that they will invoice for a cancellation? Seems wrong that crew have to bear all of the risk of changing project dates.