The sphinx stretch (or sphinx pose in yoga) is a great exercise to move into gentle lumbar extension and relax the lower back muscles.
Start by laying on your belly on a comfortable surface such as a yoga mat.
Place your elbows directly under your shoulders and prop yourself up with your forearms flat on the floor. Look straight ahead or slightly downward with a soft gaze.
Hold this position and breathe for 30-60 seconds. Relax by lying flat on your stomach. Repeat after a 30-60 second break.