People stare at the same signs the've seen on the wall thousands of times before and never think to react. They stare at the same piles of trash, walk past the same hovels, and the same wretches... And all of them think a persistent little lie which comforts them.
"I don't know what to do, so I shouldn't do anything."
That's all it takes to turn people into slaves. There's no need to extort or force. Simply make it convenient.