What tools does the Party use to maintain control in 1984? How effective are these tools?

One way the Party exerts controls, as is explained early in the novel, is to have no laws. While this may seem liberating, what it means, in effect, is that the Party can deem anything, even the smallest offense, a crime against the state. This keeps people in a constant state of anxiety that they might be transgressing. Therefore, they continually self-censor.
The Party uses constant surveillance to keep people under control. The telescreens that can never be turned off are used to spy on Party members. Helicopters hover, peering into the windows of flats. Children are encouraged to report transgressions by their parents—this happens to Parsons. Husbands and wives are encouraged to spy on one another. Anybody could be a Thought Police officer in disguise.
Propaganda is constant and incessant. People are constantly being told that life is getting better and that rations are increasing while life remains miserable and rations are cut. Big Brother is continuously extolled as all good and all wise. History is endlessly rewritten to conform to the Party's version of reality.
Violence is central to this society, from the constant warfare to the public executions to the Thought Police. The Ministry of Love is known to be a place of torture. People are also encouraged to act out their aggressions against enemies of the state through the frequent Two-Minute Hate sessions.
These forms of control are effective, keeping Party members intimidated and docile. Anyone who deviates in the least bit from orthodoxy is severely punished.

In 1984, the Party uses a number of techniques to maintain control. Telescreens and hidden microphones, for example, monitor every conversation and movement of Party members, ensuring any hint of rebellion is detected by the Thought Police as soon as it occurs. In addition, the telescreens act a visual deterrent and a reminder that Big Brother is, quite literally, always watching.
The Party also uses violence and torture to maintain its grip on power. Fear of being taken to the Ministry of Love (Winston calls it being "vaporized") keeps Party members in a permanent state of fear. Winston's reluctance to write in his diary in the opening chapter is a good example of this fear.
These techniques, then, are extremely effective in maintaining the Party's control, as shown most clearly through the failure of Winston and Julia's rebellion, which ended before it really began. Moreover, the Party's ability to "re-educate" Winston and Julia demonstrates just how adept the Party is at controlling the minds of its members.