In one word, fear. Fear is the way in which the totalitarian government of Orwell's dystopian world maintains control over their populace and forces them to comply to their commands and wishes. Throughout the novel, it is clear that both Julia and Winston are constantly alert to the fact that they may be being watched, and this vigilance is something that is associated with the Thought Police, whose job it is to constantly search out Thought Crime, where even your thoughts can betray you. The centre of this fear is of course The Ministery of Love, which is ironically named, for its real job is, as the opening chapter informs the reader, to maintain law and order. Note how the Ministry of Love is described:
The Ministry of Love was the really frightening one. There were no windows in it at all. Winston had never been inside the Ministry of Love, nor within half a kilometre of it.
As the reader and Winston discovers later on in the novel, the Ministry of Love is really a torture centre where people like himself are taken to be broken. This is not the only fear that exists, however, as it becomes clear that this is a world in which frequently people who are viewed a being dangerous in any way are just made to disappear without trace suddenly. Fear then is the way in which the totalitarian government of this dystopian world maintains control and order.