Stretching your entire body and strengthening the muscles of the abdomen that support the back are key, but any advice is a crapshoot without knowing precisely what the cause of the pain is. The most common causes are muscle spasms (mot commonly due to postural defects or disc herniations) and arthritis of the facet joints between the vertebrae. Many other, less common things can cause it. If you have pain radiating through your butt into your leg or are getting numbness and tingling in the leg or foot, simple muscle spasm is unlikely as those symptoms are strongly suggestive of nerve compression by disc or bone; if that's the case see a back specialist (my recommendation is a neurologist, because this a someone who neither operates nor manipulates and will gain nothing financial by any of the remedies/therapies he or she will recommend, unlike a surgeon or chiropractor). If it's simple back pain, try and readjust you posture. If you can find someone in your area who is a therapist in the Alexander technique, get a consult from that person who can adjust your breathing and posture. If it persists, see a specialist per above.