The pilot reported that the glider was towed to an altitude of 2,000 feet above ground level, released, and then flown for more than 2 hours without incident. He was subsequently 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-per-minute increase in lift and elected to continue the left turn. However, when the glider turned back toward the ridgeline, it encountered a large downdraft and loss of airspeed. The glider stalled, spun to the left, and struck trees. The glider's tail boom separated and both wings were substantially damaged. The pilot reported that there were no mechanical malfunctions or failures that would have precluded normal operation. He further stated that he should not have continued the left turn and that standard training techniques called for continuing to perform "S" turns when close to a ridge or mountain until the glider reaches an altitude of at least 200 to 300 feet above the highest point to provide adequate altitude to recover from possible stalls caused by downdrafts.