Doing business is not about being aggressive or having terms that are not on arms lengths to your customers. Escorts should understand they are running a business and a service, where client satisfaction is very important. Escorts should try to communicate there terms and conditions ahead of clients in a friendly manner, for example you might say 'please let me know if you cancel or change your mind one or two days before hand'. If the client cancel last minute you might say 'this cancellation is really unacceptable given I could have other bookings', you might then want to give them another chance as a gesture of goodwill. If the client flakes again then simply say you can no longer do business with them.
Making the client experience better is a win-win situation, not only would you have repeat clients but word of mouth is actually quiet big in this business.
There's different ways in approaching situations like these. I for one would never pay the full amount upfront.