How are fuel tanks cleaned out?
FAA design regulations require that the aircraft be equipped with fuel system drain valves that allow the entire fuel system to be drained. 

"Fuel System Drains - 
Aircraft fuel systems must be fitted with at least one drain to
allow safe drainage of the entire fuel system with the airplane
in its normal ground attitude."

According to a maintenance person: "The residual is vacuumed or absorbed and the negative ventilation is applied. You can make entry almost immediately, though air monitoring is constant. We keep air moving through the tank anytime there is someone in the tank. "

Doesn't clearly answer the question, but this article discusses the procedures for personal protection while entering aircraft fuel tanks.