The heart of the reign of the pigs is rooted in propaganda. Since Snowball is kicked out early on, it is really Napoleon and Squealer who spin the propaganda. Here are several examples.
First, whenever something bad happens, Napoleon has a perfect answer: Snowball must have done it. This makes the animals hate Snowball and fear him. He is a boogieman of sorts who can absorb all the blame. In this way, Napoleon is never at fault. Hence, Napoleon has a perfect solution in vilifying Snowball.
Second, Napoleon uses fear to keep the other animal in check and to justify whatever he wants. For example, the pigs take the apples and milk. They say that they actually don't like apples and milk, but they are consuming them, because science proves that these things are good for pigs. If the pigs did not take this sacrificial action, Mr. Jones would come back. Of course, this does not make sense; it is propaganda after all. This idea of Jones coming back is a refrain.
In the end, the propaganda is so powerful the animals are willing abettors of their own subjugation. Moreover, most of them believe that their lives are better.