A lot of us start to hunch because we're working on a laptop or looking down at our phones all day. The writers at the Natural Posture explain that poor posture—sitting with your shoulders forward—causes the soft muscles in your chest to get tight, which in turn makes it feel uncomfortable to sit up straight.
Wearing a brace helps get your muscles back on track by forcing them into the correct position and making you aware of how you're sitting. But you should start slowly. The Somaz brace I tested suggested wearing it for 10 minutes a day to start, as did the Upright Go devices. It may hurt at first, but slowly retraining your muscles will make it so that sitting upright is second nature. Regular exercise can also help. No matter what, go slowly and don't overextend your body. If you have serious injury or pain, consult a physician.