This thoracic-opening exercise can make a big difference in your hunched-over posture. It promotes extension in the thoracic spine, which is usually hunched or rounded forward in flexion with kyphosis.
Stand facing a table or counter that's about waist to chest high. Place your hands on it and walk your feet back so that your feet are under your hips.
Look at your feet and make sure they're straight or even slightly pigeon-toed or turned inward (this is really important). Kick your buttocks/hips back to place an arch in your lower back and move your chest down towards the floor. Keep your elbows locked out and tighten your quads.
Breathe and hold for 1 minute. Make sure to keep the weight distribution in your feet equal.
Repeat daily.
You'll need more than one stretch of course to fix poor posture, but this is a good start. See the related links below for more info.