One strategy Napoleon uses is to assume control over the laws of the farm (the commandments). He even rewrites history to make himself look better and to demonize anyone who would challenge his authority. This is why he spreads the rumor that Snowball is a traitor and was not actually a hero at the Battle of the Cowshed.
Napoleon essentially uses and changes the laws to suit his growing greed. Using Squealer as a talking head, Napoleon is able to use propaganda to influence and brainwash the animals. The initial commandment about sleeping is that no animal shall sleep in a bed. The pigs begin sleeping in beds, so Napoleon makes arrangements for the commandment to be changed, but he acts as if it had always been this way. Squealer spreads propaganda that the commandment has not been changed.
"You have heard then, comrades," he said, "that we pigs now sleep in the beds of the farmhouse? And why not? You did not suppose, surely, that there was ever a ruling against beds? A bed merely means a place to sleep in. A pile of straw in a stall is a bed, properly regarded. The rule was against sheets, which are a human invention.
Napoleon also uses fear and violence to enforce his policies. He adopts nine puppies which grow up to become his personal bodyguards. He even goes so far as to execute animals that challenge him. With some animals becoming frustrated at Napoleon's rule, Napoleon, sensing that there might be a second revolution against him, executes those who oppose him and even coerces some animals into confessing; this is an allegorical reference to the "Great Purge" in Russia in 1937-38 when Stalin got rid of his enemies.
In the end, the animals continue to be manipulated by Napoleon's oppressive rule because the continue to believe they are still better off under Napoleon than they were under Jones.