What is the best treatment to get for correcting long term postural issues?

I agree with WVPilates, Pilates is a great form of exercise to correct postural problems. I would also suggest Alexander Technique and physiotherapy [and you have rightly identified them as the right treatments in your questions] to deal with any muscular issue and osteopathy or chiropractic for spinal/joints problems (these techniques are very effective for cases of rotated pelvis).
You mention you have one shoudler lower than the other which may be a sign of scoliosis or curvature of the spine (this would also affect your hips). X-rays will show if that is the case. I had scoliosis as a teenager (well, I still have) and in extreme cases it gets corrected either by surgery or a brace (I had both). However you say that these problems have developed in the last few years so habits and posture may be the key causes. It would still be worth investigating if there is any underlying problem with your spine.
All the best,
Paola

perhaps you have acquired scoliosis/: carrying heavy loads on one side, uneven gait, tightened hip flexors on one side, a number of things can cause shortening of muscles on one side and can cause acquired scoliosis, which can be corrected through pilates, if it is from birth , you can work on balancing the muscles , although the scoliosis will still be there, you wiill be working on improving the tighter sidde of the back and the weaker side too; For a free evaluation and pilates class email me: keahlan@mobilehealing.co.uk

With the correct course of physiotherapy combined with home exercise, I've had experience in postural re-alignment working hand in hand with my colleague David @ Sixphysio in London. If you're interested in combining physiotherapy with home exercise, supervised by a personal trainer ( such as myself ), I'd be happy to help.

Without seeing you it is difficult to say, but it sounds like you have shortened (contracted) muscles on one side of your body and elongated (lengthened) muscles on the other side due to the postural imbalance. You may want to try remedial massage whereby we look at the whole person, taking into account muscular imbalances through posture and correct these first rather than treating the symptom first; then provide you with support you can carry out at home to re-dress the balance. This is a two-way system with the client and the therapist working together. Isolating muscles by stretching then strengthening them will help in the long-term too. This won't correct overnight as it has taken a while to get to whatever stage you are at, so be patient and things should improve with the support. All the advice you have been given by others is right, you just have to choose what is right for you and your lifestyle. Good luck.

Chiropractic of course.
Feel free to call reception on 0207 4197900 and I'll call you back to discuss.
Regards,
Michael H SMith

Clinically, I have found that in order to make permanent corrections, you need to address all of the components in your body that affect movement, your muscles, your bones, your nervous system, and your ligaments.
In our office, we do extensive analysis on the spine, nervous system and posture.
After our analysis we provide our patients with very specific chiropractic adjustments, exercises, and mechanical traction in order to correct the problems. After a course of treatment we re-analyze to determine what corrections were made.
If you are interested in learning more, or finding a practioner in your area, I encourage you to visit http://www.idealspine.com
Good luck!

Pilates helps with postural imbalances. Correction would depend on the cause of imbalance, your age and your determination in changing your posture. Of course, it also depend on how bad it is. You can always make improvements tough. If you find a good cliical Pilates instructor, one will be able to guide you through exercises. Good luck!