Nothing worse than an attractive and hot dead fish.
I have in the past, while in the room and alone with the girl, after she has stashed the cash, offered extra cash, to be paid at the end of the meet, directly to her, if she smiles and makes me feel welcome.
This is only with the cheaper Eastern European girls. I (generally) find the more expensive girls tend to be more professional and understand the whole ‘service’ nature of the meet, as opposed to the ‘action’ only part of a meet.
Obviously I always arrive clean and be polite as well.
I have had a good deal of success.