This stretch is so important for shoulder flexibility as well as posture. I know that I have the worst posture when I sit in my desk chair, so taking a moment to perform this move helps get me back on track!
Sit tall in your chair and reach your left hand behind your back, between your shoulder blades, palm out. Then reach your right hand up and over your right shoulder and try to touch your left hand. Hold for 10 seconds. Switch arms and repeat.
To make this move easier, grab a towel or shirt and hold an end in each hand. "Walk" your hands as close as you can.