Plato Nails This One
How a state gets transformed into an oligarchy:
SOCRATES: That storehouse filled with gold we mentioned,169 which each possesses, destroys such a constitution. First, you see, the timocrats find ways of spending their money, then they alter the laws to allow them to do so, and then they and their women disobey the laws altogether.
ADEIMANTUS: Probably so.
SOCRATES: Next, I suppose, through one person seeing another and envying [e] him, they make the majority behave like themselves.
ADEIMANTUS: Probably so. SOCRATES: After that then, they become further involved in moneymaking; and the more honorable they consider it, the less honorable they consider virtue. Or isn’t virtue so opposed to wealth that if they were set on the scale of a balance, they would always incline in opposite directions? [551a]
ADEIMANTUS: It certainly is.
SOCRATES: So, when wealth and the wealthy are honored in a city, virtue and good people are honored less.
Readings in Ancient Greek Philosophy: From Thales to Aristotle (p. 374). Hackett Publishing Company, Inc.. Kindle Edition.
The US managed to subdue it's oligarchy once. The method: tax the hell out of them, especially with inheritance taxes.