"Proper" posture is NOT forced and doesn't involve you sticking your chest out. This is a big misconception and I want to clear it up.
Proper standing posture involves your head being held over your neck, the ear canal being in line with the middle of your shoulder when seen from the side, shoulders relaxed with the arms hanging to your sides (not forced downward with the chest sticking out!), pelvis relaxed and slightly tilted forward (your belly may hang forward but don't suck it in!), knees directly over ankles, and toes pointing directly forward (not pointing in or fanning outward). Ok, I know this sounds like a lot so here's some pictures to illustrate the do's and don'ts...
If you're sitting at a desk the monitor/book should be at eye level and the keyboard should allow your elbows and wrists to be at the same level. Your chair should be directly underneath you with your bottom in the center of the chair and then the height of the chair should be adjusted so that your feet sit comfortably on the floor in front of the chair (no dangling feet, knees bent above your hips, or feet tucked the chair). If you are having difficulties or lack equipment to support these healthy workplace habits talk to your supervisor about obtaining monitor stands, new chairs, or foot rests in order to prevent injuries such as neck/back pain, disc herniations, shoulder injuries, and even carpal tunnel. I offer ergonomic consultations for workplaces in order to support healthy habits for workers and to prevent injuries in the workplace which not only cuts down on time off due to injury but also decreases worker's compensation injuries as well. (It's a Win-Win!)
Proper sitting posture involves your head being held over your neck, your ear canal inline with the middle of your shoulder, shoulders and chest relaxed (not pressed down with the chest sticking out), pelvis slightly tilted forward (not tucked or over-tilted), knees below or at level of the hips, and feet placed on floor shoulder width apart, toes pointing forward.
Again, I'm going to stress TALK TO YOUR SUPERVISOR if you're having a hard time achieving this position. Or better yet, have your supervisor call or email me and I'd be happy to schedule a consultation for your workplace.