How can the nation avoid a return to tyranny?

The United States Constitution is structured specifically to prevent the creation of another tyrannical government. For example, the modern United States political system emphasizes a separation of power between the branches (legislative, executive, and judicial) of government. Additionally, the federal government (national government system) is separate from state governments.
Another innovative feature of the American political system is what's called checks and balances. This type of federal system separates the executive branch (the president and cabinet), the legislative branch (the Senate and the House of Representatives, which together make up Congress), and the judicial branch (the United States Supreme Court and lower courts).
An example of the branches "checking" each other the president vetoing a bill passed by Congress. Subsequently, Congress can override the presidential veto if they so choose. This system of checks and balances lessens the powers of a president, at least in theory.
Additionally, the United States Constitution protects the rights of citizens to bear arms, form militias, publish criticisms of the government, and perform public protests. Each state government can also make sure that the federal government does not violate these rights. On the other hand, the federal government can "check" state governments who cause civil discord, as exemplified in the Civil War.
http://ccd.indiana.edu/file_download/27/vchecksandbalances_12.pdf