I always had bad posture: I never seat straight, I slouch badly...All that caused some severe back and shoulder pain. I sought advice of my fitness trainer - she suggested Pilates.
Pilates is all moving efficiently. What makes Pilates so important with respect to posture is that Pilates trains us to develop and use core strength, rather than holding our position with superficial musculature.
Using the deep core muscles of the powerhouse - the abdominals, back, and pelvic floor - to support our posture allows the shoulders to relax, the neck and head to move freely, and relieves stress on the hips, legs, and feet.
Most of us know good posture when we see it, and we are inspired by how free and strong it makes a person look, but there are so many reasons to attend to ones posture:
• pain relief throughout the body, including back and neck pain, hip pain, leg and foot pain.
• allows us to move efficiently
• improves muscle function
• increases range of motion
• takes pressure off of compressed organs
• improves circulation
• creates a trimmer appearance
• radiates an attitude of confidence.

Here is the alignment check list I have to follow now, to work on my posture. Best is to repeat these moves as many times during the day as possible, especially after worm up or even after workout.

1. Begin standing. Stand with your feet and legs directly under your hips, with your feet parallel, pointing forward. Knees are straight but not locked.
2. Balance Your Weight. Adjust your body so that your weight feels like it is falling directly through the middle of the foot. A good way to do this is to rock slightly forward and back on your feet, making the movement smaller and smaller until you feel your weight is balanced over the center of your foot.

3. Activate your core muscles. Lightly pull your abdominal muscles in and up. As you do so you engage the pelvic floor as well. You are going for a feeling of aliveness in the core. Just this move is usually enough to improve one's posture significantly.
4. Drop the tail bone. Activating the core will allow you to drop your tail bone directly down toward the floor. This is a neutral spine position, where the natural curves of the spine are present without tucking or hyper-extending(sway back) the pelvis. A popular image is that the pelvis is a bowl of water and you don't want the water to spill out to the front or the back.
5. Relax and open your chest. The chest is not caved in and not thrust out, just resting easily. There is a small point at the bottom of your sternum, and that, like the tail bone, should be pointing straight down.
6. Shoulders down, broad back. Allow your chest to drop and open as your back expands. As this happens, your shoulders drop away from your ears and your shoulder blades slide down your back. Cultivate a posture whereby the your core is holding you up, not your shoulders!
7. Ears reach for the sky. The head and neck are now completely supported by the core and easily float above the shoulders. Imagine that the tops of your ears are reaching for the sky. Your gaze is straight forward, with the throat open and the chin resting naturally.
8. Review the line up. If you were seen from the side your body part line up will look like this:

•ankles
•knees
•hips
•shoulders
•ears

I hope to see the results in a couple of weeks. Meanwhile I work on developing the core muscles and general awareness by...sitting on Pilates boll at work ;)
I'll be reporting on my progress shortly.