a good stretch is the "seal stretch" (don't know the real name of the stretch)
what you do is lie face-down on the ground, then place your hands on the ground somewhere around the sides of your abdomen. you simply push yourself up into an "J" shape position where your legs stay on the ground while your torso bends up and back as much as you can, like a seal on land
normally, this stretch is mainly for stretching out the abs, but I've found it to help with the lower back.
if all else fails, schedule a session with a chiropractic. their specialty is the spinal column, along with many of the muscles around there. maybe you just have something that needs to be popped into place? either that, or a back massage by a hot Swedish masseuse. I'm sure you'll forget about having lower back pains then