The Sith were clearly on top of things by the conclusion of ROTS, but I believe the "balance of the Force" was manifested in Vader/Anakin himself, i.e. when Darth Vader found the Jedi Anakin Skywalker deep within himself and killed the Emperor to save his son.
The balance was achieved when the Sith were destroyed, and overthrown by Jedi who were compassionate and selfless (unlike the Jedi Order of the Old Republic, which seemed to deny themselves their "humanity", which I believe was something that Anakin struggled with).