Its sophisticated surveillance technology makes it easy for the Party to control it citizenry. The telescreens that can never be turned off and which broadcast into private homes are two way: they can both be watched and used to watch others. Nobody knows at any given moment if they are under surveillance. This threat of the panopticon or all seeing eye is enough to cause people to self-regulate and self-censor:
You had to live—did live, from habit that became instinct—in the assumption that every sound you made was overheard, and, except in darkness, every movement scrutinized.
The Party also has the ability to vaporize people so that any trace or memory of them is wiped out of existence. Winston's job, in fact, is to rewrite history, in part in order to expunge the record of anyone considered undesirable. These people are whisked away into the Ministry of Love, where advanced torture technology through drugs and machinery make it possible to remold a person's thinking and allegiance to the state.
Surveillance even extends into Prole territory: Mr. Charrington is actually an agent of the secret police and the apartment over his shop that Winston and Julia rent is bristling with surveillance equipment.
However, all this surveillance technology is enhanced by the government's ability to convince Party members to spy on each other and report any deviance. Were people to rebel and refuse to collude with the government in surveilling each other, the Party would have a harder time maintaining control, despite its technology.