Back squats are great too, however from a posture standpoint the increased hip stretch and increased core activation make it a clear winner when it comes to improving posture.
The Barbell Shrug
The barbell shrug, specifically targeting the traps is a great exercise for correcting hunched or internally rotated shoulders.
When performing the shrug ensure you're shrugging the weight up and slightly back before squeezing tightly for a second.
Don't roll your shoulders over and over, this is the most common mistake I see being made on the shrug, as this only places stress on your tendons and ligaments - you're not actually increasing muscle activation.
Cable Face Pulls
Just like the barbell shrug, the face pull works the rear deltoids and traps.
By focusing on pulling the weight back and maintaining correct shoulder position you'll be strengthening the muscle that're responsible for your internally rotated shoulders while also practising good posture (chest forward, shoulders back - squeezing the weight on the cable machine with every rep).
Bodybuilding & Bad Posture
A key point to takeaway here is that weak muscles are often responsible for bad posture - how often do you see a guy with a physique carved and granite sitting with internally rotated (hunched) shoulders? I never have.
By following a regular workout regime, ensuring you hit exercises that strengthen key muscle groups responsible for your posture (e.g. your core, shoulders and lower back while also working on your hip flexion) i'd be surprised if you find yourself utilizing bad posture.