Well, his is a dictatorship which controls the media, the economy and the political system. It does not allow dissent, except sometimes for show. He has hundreds if not thousands of political prisoners, and has sent many of his criminals and opponents to the United States. Some people also suggest he has a very effective intelligence network among Cuban exiles living in the US.
But the largest reason for way he has been able to stay in power is, in my opinion, US foreign policy towards Cuba and the embargo. Since it economically hurts the people there, he can blame every economic ill there is (including those caused by an inefficient and outdated socialist system) on the United States. He makes a great show of demonizing the US at every turn, and for the most part, people believe him.