The first way is what you fear. People in government who want increased power to serve their own purposes and agendas. This is subversion of democracy by a few.

The second way is when the public at large becomes incapable of policing and regulating themselves. "Common sense" laws are past to force people to act in a way that they should be acting to begin with, but do to social and/or intellectual breakdown they can't or won't. IMO, this is the most serious threat to democratic ideals.

In order to have a stable and healthy democracy, the population must be intelligent, have self restraint, and take pride in themselves and their community. Without these traits, the body politic devolves into an unruly mob that must be brought to heel by increasingly authoritarian means. Freedom, like power, comes with certain responsibilities.