Checks and Balances
The three branches of government are limited from gaining too much power from the branches themselves. Each branch can halt the other two from making an unethical decision, which naturally balances things out.
The United Kingdom utilizes this as well between the monarchy and parliament. The monarchy appoints the parliament, and the parliament keeps the monarchy from doing certain things, i.e. the Magna Carta. This system keeps everyone in line, generally.
http://www.congressforkids.net/Constitution_checksandbalances.htm
http://americanhistory.about.com/od/usconstitution/a/checks_balances.htm
The United Kingdom utilizes this as well between the monarchy and parliament. The monarchy appoints the parliament, and the parliament keeps the monarchy from doing certain things, i.e. the Magna Carta. This system keeps everyone in line, generally.
http://www.congressforkids.net/Constitution_checksandbalances.htm
http://americanhistory.about.com/od/usconstitution/a/checks_balances.htm