How is District 12 controlled by the Capitol in The Hunger Games?

The Hunger Games themselves are the primary means of control used by the Capitol. By forcing each district sacrifice one girl and one boy each year, the denizens of the Capitol position themselves as godlike, having the ability to take away life as they see fit. They do all of this for the entertainment of those living in the Capitol, further diminishing the value of the lives of those in District 12. The effect this has is horrifying:

To make it humiliating as well as torturous, the Capitol requires us to treat the Hunger Games as a festivity, a sporting event pitting every district against the others. The last tribute alive receives a life of ease back home, and their district will be showered with prizes, largely consisting of food. All year, the Capitol will show the winning district gifts of grain and oil and even delicacies like sugar while the rest of us battle starvation.

District 12 is also kept very poor. They struggle each day just to survive, which is how Katniss finds herself relying on her illegal hunting skills to provide food for her family. People who are poor and near starvation have precious little energy to form an uprising against their living conditions, which is what the Capitol relies on. Katniss reflects,

Starvation's not an uncommon fate in District 12. Who hasn't seen the victims? Older people who can't work. Children from a family with too many to feed. Those injured in the mines. Straggling through the streets . . .

The districts also have no means of communicating with each other. District 12 really doesn't have any means of finding out what other districts could be planning or how they are faring. District 12 knows that some other districts have at least slightly better living conditions than they do, based on the tributes they are able to send each year, but the credibility of any other information they receive is uncertain. District 12 is physically isolated from the other districts, so obtaining any sort of information about current events (especially in the absence of almost all technology) is nearly impossible.
If things get out of control in District 12 or if the Capitol senses an uprising, the (ironically named) Peacekeepers are sent in to take needed control back. They operate as an extension of the Capitol, taking lives without mercy.
All of this is primarily dependent on keeping the citizens of District 12 in fear of the power of the Capitol, which has proven very effective for the past almost seven decades.