A PRIVATE PILOT ENROUTE TO OSHKOSH, WISCONSIN, WAS MAKING AN INTERMEDIATE REFUELING STOP WHEN HE NOTICED AN ABNORMALLY LOW FUEL FLOW, FOLLOWED BY A ROUGH RUNNING ENGINE. THE PILOT INITIATED EMERGENCY PROCEDURES TO RESTORE NORMAL ENGINE OPERATION TO NO AVAIL. THE ENGINE STOPPED AND THE PILOT INITIATED A FORCED LANDING TO THE ONLY AREA THAT WAS CLEAR OF HIGH VEGETATION. THE AIRCRAFT LANDED HARD IN ROLLING TERRAIN. THE ENGINE STOPPAGE WAS CAUSED BY FUEL STARVATION AS A RESULT OF A BLOCKED FINGER FUEL STRAINER IN THE FUEL SERVO UNIT BY AN UNSPECIFIED CONTAMINANT.