Cobra Pose.
Bhujangasana, the Cobra Pose, helps in increasing lung volume, stretching the thoracic muscles as well as in strengthening your abdominal muscles and improving posture:
Lie on your belly and inhale deeply. Slowly raise your torso while keeping your lower body to the floor at the same time. Balance your body weight on your legs and arms. Look up while maintaining your position.
Slowly breathe out then relaxing.
You can increase the duration of time you stay in the pose as you get better.