Arms straight, hands shoulder width apart, with the shoulders directly above the hands. Legs should be straight and together.Unless you have a spine that freely hyperextends, it will take some months to develop a significant arch. Focus on elongating the spine and stretching out the abdominals, as well as developing active flexibility in the spinal erectors and glutes. An example of an active flexibility exercise would be a superman.
A few exercises to improve your upper body mobility, and in turn, help improve our posture and performance.