Sitting all day at a desk is terrible for you. It puts strain on your neck and shoulders, gives you lower-back pain, and is slowly killing you. The simplest answer would be, "Well, don't sit all day, dummy!" except most people have important things called jobs that require them to stay sedentary in front of a desk to collect a paycheck and avoid getting fired.
It's not all doom and gloom, though. There are some stretches that can help balance everything out, and relieve some of the stiffness and pain that come along with sitting all day. There are even a few exercises that combat all that idle time on your ass, in front of a screen.
But there's one move in particular that's like the holy grail of sedentary-fighting workouts. It's called the Thoracic Bridge, and it opens up your hips, stretches out your lower back, and can really help with posture and balance.