The pilot reported he landed at the airport without incident. During engine shutdown, he applied rotor brake to slow the main rotor blades. When the pilot released the brake, he heard a bang and felt an impact to the aircraft on the rear left side. He reapplied the main rotor brake intermittently until the blades stopped. The pilot exited the aircraft and noticed a tail boom strike occurred. He stated he input aft cyclic during landing to accommodate landing slightly downhill and did not place the controls in neutral during shutdown. According to the manufacture's shutdown procedures, the cyclic and pedals should be in the neutral position.