After some civil strife with the Tyrants of Athens, the Spartans and many of his own fellow Athenians, Cleisthenes rose to a position of power and influence. He did so with the citizens of Athens at his back.
Once in a position of power he began the overhaul of the Athenian constitution. This had been going on for some time, the tyrant Pisasstratus having done a good bit himself. But Cleisthenes took those reforms even further, breaking the hold of the clans on the governance of the city, extending the franchise, and making it a true representative democracy (well, for male Athenians). Public servants were chose by a lottery system.
So thank this old bird for your Democracy. Well if you live in such a country...otherwise, thank him for mine!