A lot of people try to stretch their lower back out when it is tight and it keeps coming back. That is because most lower-back tension comes from tight hips, so my recommendation for you is that you gently open and realign your hips. Your back sits on your hips and if your hips are tight it pulls on your lower back. Try the hippie. It is a great stretch to do in the middle of your day or on the back of an airplane if you feel your back tightening up.
With your feet flat on the ground, slowly bend forward at your waist walking your hands down your legs as long as it feels comfortable. Hang down and alternate bending one knee and keeping the other leg straight (but still keeping your feet flat). Let your head dangle down, releasing all your tension. Stretch each side holding for 15 seconds.