I had a couple of gentlemen I'd seen regularly for quite a while come out of the blue during the holidays and, unasked, paid for sessions to be scheduled later, sort of a Christmas bonus, I guess. It worked out fine, although I did offer a refund to one fellow when conflicts with our mutual schedules delayed us for a few weeks, but he refused. I had a "mistress" arrangement with another fellow from the old ASPD days who gave me an allowance at the beginning of every month for an agreed-upon number of visits, and until he moved out of state, that worked, but again, I didn't solicit it, just went along with the arrangement he wanted. I've never had to ask for an advance from a client to help me out of a jam, so as Naomi said, I must be sucking right.
On the whole, for reasons stated already, probably not the best idea, but I am toying with a "pre-paid pleasure plan" at the request of some gentlemen, and I know another lady on here who's implemented one. The concept of a "gold pass" for unlimited play at an amusement park has been around since Mickey Mouse, so as long as the operator doesn't have a Goofy reputation, board the ride. (If you meet the minimum height requirement. )