Bhujangasana is a belly backbend which primarily opens the upper back as opposed to mid and lower back. Backbends are more about repetition than holding the pose. When practicing this pose do more repetitions rather than just one held for a long time.
Inhaling, press down into your hands, keep the pubic bone on the floor, and lift the torso up, opening the chest. Extend the front of your body to lift your chin. Move the shoulder blades down and apart.
Move the kidneys into the body as your reach your side waists forward and up. Do not strain. Keep the elbows bent or move the hands forward in space (away from your hips) to help open the chest. Lift the bottom of your belly to keep the low back from compressing.
Stay for five to ten breaths, coming down on the exhalation.
Rest with your forehead on the floor or on your hands for at least one full breath and repeat.