Head to Knee -- Janu Shirshasana
Stretches and opens back and hamstrings, improves flexibility. Sit on the floor with legs extended in front of you. Bend one leg, bringing the heel of the foot as close to the groin as possible. You may want to place a pillow under the bent knee for comfort.
Make sure your sitz bones are firmly grounded on the floor and that your spine is straight. Turn your body slightly so you face out over the extended leg. Inhale and raise your arms over head. Exhale and begin to move forward slowly. Try to keep the back as straight as possible. Instead of bending at the hips, focus on lifting the tailbone and rolling forward on your sitz bones.
Inhale and lengthen and straighten the spine. Exhale and roll forward, however slightly. To get a bit more forward movement, engage your quadriceps (thigh muscles) as you move forward. This releases the hamstrings, giving you a bit more flexibility.
When you have moved forward as much possible as you can, lower the arms and grasp your foot. Hold the position for a moment and breathe. Then on the next exhale gently pull yourself forward. Go slowly and remember to keep the back straight. When done, straighten up and do the other side.