All of the suggestions here are good information, especially woofgang's requirement that you see a doctor. However, for long term releif (unless it has a cause such as described by WendyS, is to become involved in physical activity that includes stretching of the hamstring muscle and ligaments. Our modern life style allows the muscles to "shrink" up and they are always involved in lower back pain. Lying on the back and raising the straightened leg with the toes pointed down towards you is a good start. Simply bending and reaching for your toes with outstretched hands is also adequate. For the latter, work your way up to assuring the legs are straight and locked at the knees. This will, most assuredly, take a period of time to become comfortable, but should notice some relief almost immediately. Be sure, as stated, to get advice from the doctor first...