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Give them hope and a way out.

First, have very strict laws on what can and cannot be done to the slaves. Knowing that they cannot simply be tortured for the sadistic pleasure of their owners or simply killed for no reason would eliminate a great deal of anxiety.

Second, have an incentive for the slaves to work hard. In ancient Rome, slaves were paid very modest wages, could own property of their own, and even buy their own freedom. Have pay rates higher for the slave positions you want filled. Having them see a way to better their position would put them in a far better mood and motivate them to excel.

Third, the slaves of a more carnal nature should be given a very high status and limited period of servitude in that role. Strict laws against disfigurement and physical abuse should apply here. The exception to this should be that any slave that harms another slave would be sentenced to this role without any protections and may even be tortured to death for the pleasure of its master.

Any harm to the masters done by a slave should have as its penalty, a quick execution.

Laws should be in place for the confiscation of poorly treated slaves. Any owner not taking proper care of slaves will have slaves removed and placed with new owners.

Lastly, all working slaves should be provided with the necessities of food, shelter and clothing with the opportunity to use their wages to buy up into better conditions. Slaves would initially live in a common area but could buy their way up to private apartments and eventually private housing. Buying better conditions rather than staying in the common area would also prolong the amount of time it would take to purchase their freedom while at the same time improving morale.