What stops one branch of government from becoming too powerful, or from abusing of its powers is the so-called principle and system of check and balances.
Through this system, all the three branches of the government (Executive, judicial and legislative) oversees, limit and check each other so a balance in the government could exist.
Under this principle, each branch can respond to the action of the other, for example, in the United States, the Judicial branch has the power to declare unconstitutional any law the Legislative branch makes, as well as any of the Executive's action. At the same time, the President is the one who nominates the Justices of the Supreme Court, and the Legislative Branch is responsible for confirming or rejecting those nominees.
