In 1984 there is talk of "power over man." What is necessary to control man?

On the most basic level, Winston himself provides the grim answer to this question during his interrogation by O'Brien, saying that power is exerted over a man "by making him suffer." Though most citizens of Oceania are not made to undergo the kind of torture inflicted upon Winston, they are nevertheless kept in a constant state of fear. Everyone knows that he is under surveillance by the Thought Police. Moreover, the endless low-level wars carried out against Eurasia or Eastasia are a subtler form of control and a means by which the Party keeps the populace focused upon an external enemy. The Two Minutes' Hate similarly redirects whatever energy that remains to the average overworked and underfed person towards the "villain" Goldstein, whom the Party has set up as a phantom-like symbol of evil.
A more profound control is exercised through the Party's manipulation of "history"—assuming that in such a dystopia that word has any meaning. Winston's job at the Ministry of Truth is essentially to rewrite the historical record. The Party is continuously destroying, altering, and falsifying documents in order to expunge anything that would cause people to question its infallibility. The old documents are thrown into a "memory hole" and replaced by new ones that convey the Party's false view of reality. It is remarkable that Orwell wrote 1984 prior to the digital age, at a time when the computer was in its infancy and had not yet come into general use in business, government, and science. The falsifying process is shown being carried out with the "speak-write," evidently a computer-like device that allows Winston and the other workers at the Ministry to replace truth instantly with lies.
This last process is the ultimate form of control over man. Given that our understanding of the world is based upon our access to facts, leaders who distort those facts and replace them with lies can then manipulate the unknowing masses into blindly following their dictates. The Party creates a population of suppressed, zombie-like beings. This is the reality in 1984 represented by O'Brien's terrible metaphor of "a boot stamping on a human face, forever."