In 1984, how does the Party control the minds of its citizens?

The Party wages a ceaseless campaign to control the minds of its citizens. People quickly learn to self-censor any unorthodox ideas so as not to be arrested and accused of "thought crimes."
The Party is also working on its Newspeak dictionaries, which are meant, as Winston learns, to reduce the English language to a minimum number of simple words. This helps greatly with mind control because it reduces the citizens' capacity to formulate unorthodox thoughts.
The Party also uses surveillance to control thought. People know they are being spied on all the time, and for that reason it is best not to think in an unsanctioned way and risk showing, through a facial expression or a muttered word, that one is out of alignment with the Party's ideology.
Memory holes are everywhere, so if citizens find writing that does not conform to Party rules, they can quickly send it down a hole to be burned in a furnace. This encourages people to "forget" what is no longer considered the official truth.
Constant propaganda drums into people's minds the way they should think.
The threat of extreme punishment helps control thoughts. When Winston writes in his journal, he knows the penalty will be either death or twenty-five years in a hard labor camp.

The Party uses classic divide-and-rule tactics to control the minds of its citizens. The people of Oceania are constantly encouraged to turn against one another in this totalitarian society. No one can be trusted, not even your closest friends, relatives, or work colleagues. At any moment, someone could inform on you to the authorities for the most trivial of reasons, then you'd run the serious risk of being tortured, imprisoned, or even vaporized.
So there's a general atmosphere of mistrust in Oceania. This inculcates a mindset of subservience and acquiescence to the Party and its dictates. Because if people can't trust each other, then they only have the Party left to turn to. With all the chaos and chronic uncertainty surrounding them on a daily basis, citizens have a deeply felt psychological need for stability. That's where the Party comes in. It caters for this overriding need and in doing so successfully controls the minds of its citizens.

In 1984, the Party controls the minds of its citizens in two ways. The first of these is through propaganda: it controls of the flow and content of information and erases any aspects which portray it in an unfavourable light. We see this in Part One, Chapter Four, for instance, when Winston erases a "pledge" made by the Party stating that the chocolate ration would not be reduced. In its place, he writes that the ration will have to be reduced "sometime in April" in preparation for a forthcoming reduction from "thirty grammes to twenty."
Additionally, the Party uses violence to control the minds of its citizens. This job is carried out by the Thought Police who imprison thought criminals (people who criticise the Party) in the Ministry of Truth. From Winston's experience in Part Three, we see that the Party uses torture and Room 101 to make people believe what it tells them and, ultimately, to "love Big Brother."