Proper shoulder posture should align your back along the natural curves of your spine, and be upright over your hips. This posture is referred to as neutral spinal alignment. Working at a computer, sitting in oversized chairs, driving and leaning all create forward rounding of the shoulders. Improper muscular balance between the chest and back and poor core strength also contributes to slumping forward. Being aware of how you sit and stand, and maintaining proper muscle strength in the back and chest will help you improve your posture from your shoulders to your hips.
Find your neutral alignment. Everyone has a different neutral spinal alignment; yours follows the natural curves of your spine. To find your neutral alignment, use the "five points" exercise. Stand tall with your back against a wall, and, in this order, touch your heels, bottom, shoulder blades, skull and knuckles to the wall. Pull up tall as you go from your feet up to your head. Once you have these body points against the wall, notice the placement of your shoulders, and how they align with the natural curves of your back. This is your neutral spinal alignment.
Practice sitting properly in chairs. Sitting in chairs with bad lumbar support can cause you to release your core and slump forward with your shoulders. Use chairs that are the right size for you; your feet should touch the floor, and your back should be able to touch the back of the chair without rounding. Sit up tall by keeping your knees bent at a 90 degree angle, your feet flat on the floor and your back in neutral alignment, shoulders upright over your hips. If you are working at a desk, make sure your chair is the correct distance for you. You should not have to reach, or round forward, to access the desk or table.
Stand tall. When standing, keep your spine in neutral alignment, and if you will be standing for more than 20 minutes, switch your position. Standing for long periods of time in one position puts stress on your hips and lower back. If you will be standing for long periods, shift your position by moving your foot placement. Make sure you don't put stress on one side by swaying your hips or back. A neutral standing stance is to place one foot slightly in front of the other, and switch frequently.
Walk with a proper gait. Maintain neutral spinal alignment when walking, to keep your shoulders from rounding forward. When you walk, your shoulders will have a tendency to lean forward with your movement, so be aware of this, and re-align them over your hips -- back to proper alignment. Your arms and legs should swing naturally when walking; avoid pulling your torso forward by extending the arms too far forward.
Strengthen your core and upper body. Maintaining core and upper-body strength will keep your alignment in balance. You can do simple exercises throughout the day to help strengthen your core. When you are driving, do stop-light abs by engaging your core at every stop light or sign -- pulling your belly button toward your back and slightly tucking your tailbone. Hold the muscle contraction for the length of your stop, and maintain comfortable breathing. Release when the light turns green, or when you accelerate away from the stop sign.