The pilot reported that the glider was towed to an altitude of 2,000 feet above ground level, released, and flown for more than 2 hours without incident. He subsequently was looking for lift and performing "S" turns, while flying away from a ridgeline. During one of the "S" turns, he encountered a 200 to 300 foot increase in lift and elected continue turning to the left. As the glider turned directly toward the ridgeline, it encountered a large downdraft and loss of airspeed. The glider stalled, spun to the left, and immediately struck trees. The glider's tail boom separated and both wings were substantially damaged. The pilot reported that he did not experience any mechanical malfunctions or failures during the flight. He further stated that he should not have continued the left turn and that standard training techniques called for continuing to do "S" turns when close to ridges or mountains until at least 200 to 300 feet above the highest point, which would provide for adequate altitude to recover from possible stalls caused by wind gusts.