How to Mobilize Your Spine
To mobilize your spine around your lower and middle back, first sit cross-legged with your hands resting lightly on your knees. Then, lengthen your spine by sitting up straight with your head tilted slightly back. Inhale slowly and deeply through your nose, then exhale slowly as you turn your body as far to one side as you can while keeping your spine lengthened. Next, do the same as you turn to the other side. Repeat this exercise 10 times on each side, then sit cross-legged with the opposite leg on top and do it another 10 times.
Find a flat surface, preferably with light padding. A carpeted floor works great. Lie on your back and stretch. Imagine that you're being pulled from your feet and the top of your head. This lengthens the spine. Tuck your knees into your chest and wrap your arms around your legs. Tuck your head slightly toward your knees. Try to make your lower back curve out as you do this. Rock gently up and down on your spine, slowly increasing your motion until every part of your back (except your neck) is rocking on the ground. Move softly, avoiding hard and jerky movements. Don't rock up onto your neck. Sit on the floor with your legs straight in front of you.  Bring one foot up and rest it to the side of your other knee.  Reach across with your opposite arm and place your elbow on the outside of your raised knee. Let your forearm rest along the outside of your shin if you can. Turn your upper body and look as far as you can to the side of the raised knee, pushing with your arm against the raised leg. Hold this for 5 to 10 seconds. Repeat on the other side. Do each side 10 times, alternating sides. Remove anything from your pants pockets and sit cross-legged on the ground with your hands resting lightly on your knees.  Lengthen your spine by imagining a string pulling on the top of your head (towards the back).  Inhale slowly and deeply through your nose.  Exhale slowly and turn your body as far as you can to the side without losing the lengthened spine. Push against your knee as you turn. Repeat, turning to the other side as you exhale.  Turn to each side 10 times, then switch legs (if you sit with your right leg crossed over your left, switch so that your left is over your right and vice versa) and repeat each side 10 more times.  Stand with your feet parallel to each other, hip-width apart. Place your hands on your hips. Move your hips in a wide circle, trying to keep your shoulders above your feet (which remain planted).  Complete 5 circles in one direction before switching. Switch 3 or 5 times so that you do 10 or 20 circles in each direction. (5 is a minimum, you can do more so long as you do the same amount on either side.) Relax your neck completely, letting your chin sink to your chest.  Take your head in 10-15 circles to one side, staying as relaxed as possible.  Repeat on other side. Do both sides as desired. Stand erect, feet hip-width apart and parallel. Don't lock the knees. Imagine a line connecting the tops of your ears over the top of you head. The line should cross over the apex of your skull, which can be a little tender. Tilt your head back.  Imagine a string pulling straight up from the apex of your skull. As your head moves forward, you should feel it lift up and move back (as opposed to down and forward), pulling your shoulders back and your chest up. Remember this position as proper posture. Come up from the top of the head.  Imagine that you are coiling your spine around a rod stuck behind your jaw, trying to move a single vertebra at a time. Go SLOW! Coil as tightly as possible around that rod while staying relaxed. Let your arms and shoulders relax completely, let them hang. When you coiled as far down as you can go, slowly reverse the process. Focus on stacking each individual vertebra and keeping the others flexed as you come back up. At the top, come up from the top of the head again.  Repeat 5 times.  
Find a flat surface, preferably with light padding. Lie on your back and stretch. Tuck your knees into your chest and wrap your arms around your legs. Rock gently up and down on your spine, slowly increasing your motion until every part of your back (except your neck) is rocking on the ground. Sit on the floor with your legs straight in front of you. Bring one foot up and rest it to the side of your other knee. Reach across with your opposite arm and place your elbow on the outside of your raised knee. Turn your upper body and look as far as you can to the side of the raised knee, pushing with your arm against the raised leg. Repeat on the other side. Remove anything from your pants pockets and sit cross-legged on the ground with your hands resting lightly on your knees. Lengthen your spine by imagining a string pulling on the top of your head (towards the back). Inhale slowly and deeply through your nose. Exhale slowly and turn your body as far as you can to the side without losing the lengthened spine. Repeat, turning to the other side as you exhale. Turn to each side 10 times, then switch legs (if you sit with your right leg crossed over your left, switch so that your left is over your right and vice versa) and repeat each side 10 more times. Stand with your feet parallel to each other, hip-width apart. Move your hips in a wide circle, trying to keep your shoulders above your feet (which remain planted). Complete 5 circles in one direction before switching. Relax your neck completely, letting your chin sink to your chest. Take your head in 10-15 circles to one side, staying as relaxed as possible. Repeat on other side. Stand erect, feet hip-width apart and parallel. Imagine a line connecting the tops of your ears over the top of you head. Tilt your head back. Imagine a string pulling straight up from the apex of your skull. Come up from the top of the head. Imagine that you are coiling your spine around a rod stuck behind your jaw, trying to move a single vertebra at a time. Coil as tightly as possible around that rod while staying relaxed. When you coiled as far down as you can go, slowly reverse the process. At the top, come up from the top of the head again. Repeat 5 times. 