The pilot/owner of the helicopter stated that while in cruise flight, he let his rotor rpm decay due to inattention and "complacency." He decided he could not recover the rotor rpm "quickly enough" and performed an autorotation to a pasture. The pilot leveled the helicopter about 8 feet above the ground where it "lost lift [and] dropped straight down." At ground contact, the main rotor blades contacted the tail boom, which resulted in substantial damage. The pilot reported there were no mechanical malfunctions or failures of the helicopter.