When we're asked to do work on certain dates we usually ask for a 20% deposit to be paid in advance. Last minute bookings, and some expensive jobs may require a higher deposit.
Deposits go towards preparatory work that must be done ahead of time, pre-production expenses, and provides some compensation for subsequently cancelled bookings. Cancelled bookings cause two losses; the original job plus any other jobs that we had to reject for the same date. Once we're booked to take on a job, we don't take on any other conflicting work for the same date.
For reasons such as the above, deposits are non-refundable.