Principles of the US Constitution.
A written, entrenched constitution.
Found in one source. Entrenched meaning difficult to change (2/3 senate, 3/4 states).
Separation of powers.
Separated by three branches of government, the executive, the legislature and the judiciary. By electing them separately and forbidding the same person from sitting in more than one branch (minus the Vice President who chairs the Senate). They are given different roles and powers.
Checks and balances.
The three branches are forced to share power through a series of checks and balances, where one branch needs to cooperate with another to carry out roles. In effect there are no pure separation of powers between as one branch can limit the power of another. Checks and balances ensure that power is not concentrated and they all must enjoy widespread support to succeed. These checks however do cause deadlock, which sometimes means little gets done.