Which were the methods to control slaves in USA?

Clearly, all of the above answers are right, but there are a couple of important things that none of the three previous answers has mentioned. The previous answers all focus on coercion and brutality.  But much of the control of slaves was based on much more subtle things.

First, the slaveowners used (or tried to use) religion to control slaves.  They encouraged slaves to believe in a version of Christianity that emphasized obedience.  They encouraged the slaves to think of their rewards in the afterlife, not of their problems in this life.

Second, they did not allow slaves to learn to read or write.  This was partly meant to prevent them from reading about ideas of universal rights and things like that.  But it was also to prevent them from having the tools needed to start rebellions.

Finally, they used psychology, at least with some slaves.  Some masters treated some slaves with something approaching respect -- gave them high responsibilities and such.  This could create something like loyalty in those slaves.

I'm not saying brutality wasn't used.  But the more subtle things are just as effective, if not more so.