Make sure you make that a "non refundable deposit", and that if you aren't given 24 hours advanced notice of cancellation, you will be required yet another "non refundable deposit" if they want to make another appointment.
On that note, make sure you ALWAYS get payment when your services are rendered.
Get a contract, everything you agree to in writing and have it signed and witnessed.
And 50% payment non refundable upfront.