Aside from stretching out tight muscles, an important part of reducing back pain is to strengthen muscles — especially in your core — to lend more support to your back. Many people consider sit ups to be too intimidating or hard to do, without realizing that even a partial curl up can do a lot to strengthen your abdominal muscles and, in turn, reduce pain in your back.
Instead of doing sit ups, try strengthening abs by doing a partial curl. Start lying down with your knees bent so your feet are flat on the floor. Tense the muscles in your stomach and use them to lift your head and upper body about 4-5 inches off the ground, stretching your arms out straight past your knees. Hold for a couple of breaths, then relax. Repeat the process, gradually increasing the number of repetitions as you gain more strength.