Well, you could get an invert board... But they're pretty expensive and take up a whole lot of space!
What I do is this:
Stand in front of a counter top (kitchen, whatever). With your palms facing forward, place them on the edge of the counter. Lean forward slowly, putting all your weight on your arms/hands. As more weight is put on your palms, less weight is put on your feet, and your torso will stretch. Once you feel a good stretch in your lower back, exhale deeply. You should feel a couple "pops" in your lower back as your vertabrae expand. Feels great! Do this a few times a day (in addition to regular lower back training) and you'll feel good as new!