What methods or tactics might a modern-day leader use to try to control his or her country and people?

A leader can use physical and psychological violence, remove individual and collective liberties from citizens, instill fear and establish a government based on totalitarianism.

Explanation:

To control population, a modern leader can base his government on physical and psychological violence. The leader can exercise constant and permanent vigilance over the life of the population, controlling even the personal and family life of individuals, that is, an extreme and radical case of authoritarianism.

In addition, this leader can end the personal and collective freedoms of the population, establish a strong censorship in the media, among other measures.

Fear, its a simple and very effective tactic that can almost always work. Everyone has something they're scared of, and it isn't hard for governments to figure that out.