The concept of "Preferred Customer Status" really only works when you are offering a commodity.
Think of those TV ads for Ally Bank where new kids are treated better than current customers; current customers don't think it's fair. But that's because everyone gets exactly the same service, but for a different price. For some businesses, it's cheaper and easier to get new customers than it is to hang onto existing customers; especially if the bsuiness provides a commodity.
If you aren't selling a commodity, then most of your business will come from a very small number of clients.
I would think that an agency or massage parlour may benefit from a PCS, but not an independent provider.
Some guys like to meet a new woman every time, but some guys may be more pre-disposed to coming back to a woman they've already enjoyed being with. If a customer returns, the relationship should start to change whereby they stop being a "customer" and become a "client". There is a mutual reliance which may grow over time. How it grows will depend on the two people involved, but since it's the SP's business we are talking about, she should be the one to actively manage it.
Discounts may appeal to some; for others, the thinking and preparation that the SP puts into the encounter may provide extra value; she may initiate something new that adds excitement, or enthusiastically accomodate a special request; sometimes she may allow the session to run over; and sometimes they may enjoy each others' company over a meal, coffee or drinks.
It's a business in that it's competitive, and there are costs associated with providing the service. But it's also damned personal, on both sides.
It's up to the SP to manage the relationship from a business point of view: the guys may not always remember what you did during an encounter, but they will always remember how you made them feel.