In my experience, you don't "negotiate" to select a regular, for discounts and loyalty bonuses. You don't have a card they stamp, sixth fuck free. It will just kind of happen, as will the benefits of seeing a girl regularly.
For God's sake, don't try and negotiate it up front. "Hello, I'm interviewing candidates for the position of Regular Fuck Buddy, salary to be agreed" I suspect the kind of girl you're looking for wouldn't respond too well to that scenario. It also sounds creepy as fuck!
A true regular girl would never take the piss, as you put it. Pro$$ies do like regular clients as the risk factor of meeting someone new is removed, plus, if you click, they can actually enjoy a good session, which in turn will make it better for you.
Keep searching, you'll know when the right girl comes along.