Stretch your hamstrings by sitting on the floor with your legs straight in front of you and reaching for your toes. Do this gently, don't jerk, and hold the position for 10-20 seconds, repeat a few times. This will decrease the tightness in your hamstring muscles which is often associated with back pain.
Also, do some sort of stomach exercises, such as crunches. Weak abdominal muscles are also a source of back pain.