First and foremost, I completely respect all girls' decisions on how they choose to conduct their business affairs. I certainly don't enjoy it when people come to the car wash and tell me how to do my job so as long as you're making clear in your marketing that you do not see anyone without significant advance notice, more power to you!
That said, the answer to your questions, which appear to be two, each repeated once: 1) Just be honest, tell them that's your policy. The cool thing about being straight forward is you that don't need to wonder about what you said last time! 2) There's nothing you could say to me to encourage me to get excited about booking in the future. Indeed, what usually happens is you go straight to the 'don't bother" list, almost assuring that there will be no future appointment
I have been doing this awhile and have been burned too many times with prebooking ("Oh, I'm so sorry, something's come up," one girl was honest enough to tell me she got a better offer and others have just blown me off) so I've quit doing it except in the rarest of circumstances and since I've now seen almost all of my favorite porn chicks who escort (Someday, Taryn Thomas!), have no real reason to prebook. To cite my mantra, it's a buyer's market in this activity so if you're not willing to see me on short notice, clickity, click, click, there's someone who is who's just as cute!
I guess my point is that it seems to me that girls who do that may very well miss out on good clients. It doesn't take long to screen, especially some of the guys here who have multiple reviews and are white listed and if you get over that hurdle and are available, why say no?