According to the pilot of the Cessna 180, she was of a small stature and required the aid of additional seat cushions in order to operate the airplane. After completing an uneventful flight and landing on a turf runway, the pilot taxied the airplane clear of the area. While turning off the runway area, the nose of the airplane started to "drop." The pilot attempted to arrest the "drop" by pulling aft on the control yoke, but had to brace herself to do so. She then inadvertently stepped on the brakes, and the airplane "nosed over," resulting in substantial damage to the wing spar. The pilot reported no mechanical malfunctions with the airplane prior to the accident.