Open the sides up by bringing the tricep muscles into the back muscles. When the lift in the upper chest is felt, take the head back. Deep breathing is essential to the asana. On the exhale slide deeper into the pose. Often pressure is felt in the wrists or the lower back.
Blocks are a great help to ease that pressure.
Place the hands on the blocks as shown to get a better lift. These are just a few hints to help you enjoy the full benefits of Urdhva Mukha Svasana, the Upward Dog. It is said that you are only as young as your back is flexible. An asana routine featuring back bends is essential to a limber back. To further enhance your experience of this pose, breathe deeply while arching the spine.