Poor posture and poor postural habits were large contributing factors to problems around my neck and shoulders. The postural problems were in several areas.

1. Sitting at computer: I would slump forward, chin poked out and neck craned forward. I have observed since that this appears to be a very popular posture for computer use.
2. When writing I would lean over, bending my head down to my work.
3. Cycling, swimming: I would, once again, stick my chin out and look forward. In swimming training I was usually watching the feet of the person in front. In cycling my racing bike had a high seat and low handlebars, meaning I would lean over with a rounded back and head forward and lifted.
4. Sitting in a chair, car, sofa,..: I ,like many I see, would slump in my chair, not sitting at the back of the chair properly.
5. Whenever bending over to something I would lead with my head, stretching out my neck and sticking out my chin.
6. Sleeping I would often sleep on my stomach, with my head turned to the side. When sleeping on my side my pillow was too low to support my neck properly.

Fixing my posture, learning new habits was very difficult, and has taken me several years. As commented on earlier some of the advice I received was not so helpful. The most useful method I found was the Alexander Technique. There are people around who teach this, but unfortunately there was noone where I was. Instead I got several books out of the library on it. I will not attempt to explain the technique here, but it assists in postural retraining.

Below is a summary of the posture I learnt from the Alexander technique and other advice.

Standing: Starting at the top. Keep a little tension on pulling the chin in, but don't pull it in strongly. Think of the top of your head being pulled forward and up, try and stand as tall as you can, feel your neck as being pulled away from your shoulders. Drop and relax your shoulders, then think of your shoulders being as wide as possible (don't actively try and pull them backwards though). Keep the curvature in the bottom of your back, but don't let the hips roll forward. To assist this unlock your knees and keep them slightly bent.

Sitting: Don't cross your legs. Sit at the back of the chair. Lean back on the chair. Pull chin in. Use a lumbar support pad if necessary. Have feet on the floor.

Lying: I had to teach myself to get used to sleeping on my back, as it was too painful sleeping on my side (for neck and shoulders). Use a orthopaedic pillow (available at most shops now), lie relaxed with hands at sides, or crossed across stomach. I now sleep on my side again. But use two pillows (top one a shaped pillow), to keep neck at right height.

As a summary always think and feel tall, wide, forward, up.

The important points are to strive for correct posture, and to get into the habit of reminding oneself many times during the day to go through the postural checklist. Trying to constantly hold 'correct' posture is very difficult, and can become very uncomfortable. I found it better to instead regularly create correct posture, then relax. Keep reminding yourself, and, for example, pull your chin in and see what it feels like, then relax again. Gradually the striving for changed posture becomes more unconscious and the feelings more natural, though it has taken me several years to achieve this. Now whenever I sit down, lie down, or have a break in thought I go through my mental posture checklist, make adjustments, then relax and forget about it.

The working environment is obviously very important as well. Having a comfortable chair, a desk at the right height, etc. I put a lot of work into getting things just right.