Focus on keeping your chest up, head neutral and lower back straight. A lot of us hunch forward in our chairs and that's exactly what you DONT want to do.
a big posture problem is because of our office jobs.. we sit in a chair and tend to slump and our neck has back support. this means our front neck muscles become incredibly weak and our shoulders tend to roll forward. this is made worse in the weight room when we workout and overload our shoulders and under work our back.
a good physical therapist will teach you exercises to fix this and will also show you how to properly sit in your chair.