With poor sitting or standing posture the back the will hunch forward and become weak. This will lead to tight hip muscles to balance out your posture (right image).
The Hip Flexor Stretch helps stretch our quadricep/thigh muscle that attaches to the front hips (left image).
Here are the steps in this stretch:
Lying on your back, let one leg hang over the edge. This may be enough to stretch the hip and low back.
Hold for 30-60 seconds.
Switch sides and repeat.