The pilot departed for a short flight to his own private landing area. While approaching to land, he initiated a practice autorotation toward the hangar. During the descent, all indications appeared normal. As the pilot started the flare and increased the throttle, the low rotor RPM warning activated. He continued the flare to reduce forward speed and the low rotor RPM warning sounded again. At this point, the pilot elected to perform a run-on landing. He lowered the nose of the helicopter to level the skids. The ground was soft due to recent rain and after the helicopter skidded about 15 feet, the left skid dug into the soft ground, and the helicopter tilted forward, and the main rotor blades contacted the ground. The helicopter rolled onto its left side and the tail boom separated which resulted in substantial damage to the main and tail rotor blades. The pilot reported that there were no mechanical anomalies with the helicopter which would have precluded normal operations.