The terms are staggered into three tranches, so 1/3 of the Senate has the ability to change leadership every two years.
Election years for each senator of the same state is diffierent. Feinstein, for instance, is Class I. This means she faces reelection in 2012. Boxer, on the other hand is Class III, so she faces reelection in 2010. In this way, states continue to be represented in the Senate during any reelection year.