The pilot's selection of unsuitable terrain for takeoff, and his failure to maintain directional control after the aircraft's right main gear contacted a small dirt mound during the takeoff roll at the remote grass airstrip. Factors include the pilot's failure to walk the length of the runway during his pre-flight preparation to insure that it was clear of objects, a hidden object on the takeoff surface (the mound), high vegetation on the runway surface, and trees near the side borders of the airstrip.