How does Napoleon avoid getting caught in his contradictions?

Throughout the novel, Napoleon routinely contradicts himself by saying and acting in opposing ways. To prevent himself from being caught, he employs some propaganda and Squealer to make it look like he has not contradicted himself at all.
To see an example of this, take a look at Chapter Six and the controversy surrounding Napoleon's decision to never engage in trade with humans. Instead of admitting that he has changed his mind about trading, Napoleon, first of all, gets the sheep to start bleating so as to silence the other animals. Later, he sends Squealer out to explain that the animals are suffering from a faulty memory: Napoleon never said that he would not trade with humans.
Time and time again, we find Napoleon covering up his contradictory behavior through propaganda, like Squealer's persuasive rhetoric, designed to make the animals doubt their memories.