The pilot of the glider reported that he was taking a new glider club member for an introductory flight. He stated that the flight was "normal with good lift," and he was never more than 3 miles from the airport. He entered the traffic pattern for landing on runway 16. On final approach about 3/4 miles from the runway threshold, the pilot felt the glider sink and noted that he was becoming too low to reach the runway. The terrain off the end of the runway was covered with juniper trees and sagebrush. He turned left to land off airport in the "only open spot." The left wing struck a tree, and the right wing tip then struck the ground. The right wing and the fuselage sustained structural damage. The pilot stated that from an altitude of 1,000 feet above ground level this glider "should be able to fly 3 miles, I lost 800 feet in less than 1/2 mile."