On June 6, 1998, at 1635 mountain daylight time, a Piper PA-28-140, N4541R, registered to and operated by a private pilot sustained substantial damage while conducting a forced landing about one mile northwest of the Harrison Sky Ranch Airport, near Harrison, Nebraska. The pilot said that after the run-up he forgot to set the magneto switch to the "both" position. He said the engine was not providing enough power to sustain flight on only one magneto and he elected to conduct a forced landing in an open field. The pilot and one passenger reported no injuries. The 14 CFR Part 91 flight was operating in visual meteorological conditions. No flight plan was on file. The flight was originating at the time with the intended destination of Scottsbluff, Nebraska. The pilot said after the accident he discovered the magneto switch to be selected to only one magneto. He said in his written statement that he did not use the mixture control to reduce the rich mixture which he believed might have also reduced the power output of the engine. A subsequent examination of the airplane failed to reveal any pre-accident anomalies which might have contributed to the accident.