Control is maintained in the farm in a couple of ways. I think that seeing the post- Revolution consolidation of power by the pigs, and most notably Napoleon, is a part of this. In the haze of revolution, Napoleon takes the pups and rears them as his own. He understands in an extremely lucid manner that the only way power can be maintained on the farm is through demonstrative means that ensure obedience. Napoleon seems to adopt the perspective that such allegiance has to be imposed from "top down." It is for this reason that Napoleon rears the pups to be his own security force, whose sole purpose is to enforce the word of the boar at all costs and by any means necessary. One need only see the public executions in Chapter seven as an example of this as demonstrating how this ensures submission and control of the animals on the farm.
Squealer is another example of how control is maintained on the farm. Squealer's purpose is to "spin" political reality to a point where the animals are numb to activism and to questioning authority. Through his various tricks, he is able to maintain a sense of control over the animals in suggesting that they are better off than they were with Jones, or that the "readjustments" are for the animals' benefits, as well as other means by which the animals are placated into believing that their lives are better with the pigs' leadership. In this, control is maintained through both dulling the sense of dissent as well as ensuring that those who actually do activate their sense of voice are punished in the most severe of terms.