The pilot reported that he was performing touch-and-go landings. After the first landing and during the takeoff roll, he experienced difficulty maintaining directional control of the airplane and aborted the takeoff. The pilot taxied the airplane back to the runway and departed again. After landing, the pilot applied full power for takeoff and the airplane veered right of the runway centerline. He applied brakes as the airplane exited the runway and impacted a taxiway sign. The airplane came to rest in a grassy area adjacent to the runway and sustained substantial damage to the fuselage. The pilot stated that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation. The pilot noted that he positioned his seat too far forward which prevented him from differentiating between rudder and brake inputs. He further stated that if he had used an additional seat cushion, he could have positioned the pilot’s seat further aft and prevented the inadvertent brake inputs.