That motivating factor serves as a highly successful reward/punishment function because it internalizes rewards and punishments. In other words, you never have to ask “why” you are being rewarded or punished. It is entirely based on your own successes and failures, and the conditions are obvious.
Then there are keys rewarded for good service. Since good service is again subjective, for rote tasks like greeting guests, it’s helpful to occasionally have another slave watch and grade using a checklist. Did she say the right things? Make the proper submissive gestures? Was there any inappropriate eye contact? Was proper posture maintained? That objectivity means everyone is graded according to the same standards. Maybe a new slave is only expected to achieve 75% competence, while an experienced one has to achieve 100% to receive a reward. That can be quantified if you’re careful.
For interpretive tasks, obviously, it’s much trickier—you don’t want a blowjob from someone going down a mental checklist, it’s very boring. A simple ranking system is better, with a visible board showing who is ranked where to provide self-motivation. At the end of the week, the best in each category receive keys while the worst have them taken away.
Then you have the divisive force, arguably the most important part. There’s no point in buying multiple slaves, training them, feeding them, and so on just to have them escape into the night. Or strangle you while you sleep. Fear can keep them in line, but even better is to divide and conquer. If they’re at each others’ throats, they won’t be at yours. Up until now, we’ve been strictly, uncompromisingly fair. That veneer of legitimacy will help us when it’s time to cheat and manipulate the system for our own ends.