To swing pain free and to hold the normal curvature of the spine during the swing, you should strengthen the muscles involved with the back raise exercise. This is the only exercise that strengthens the lower back muscles through the full range of motion.
By doing the back raise you will also find that you may eliminate back problems or not create any more back problems than you typically experience. As an added bonus this exercise will help give you good posture allowing you to do many daily tasks with less effort and pain-free.
For more information on this topic, see Explosive Golf. For visual demonstrtion of this and other exercises specific to the golf swing see the Explosive Golf DVD