Bad posture

Yoga is great for good posture

Basics of improving posture

Kyphosis (exaggerated curve of thoracic vertebrae or upper back rounding); short and tight pectorals, weak and elongated traps – helped by stretching pecs, upper and mid back, traps, rhomboids and thoracic extensors ( essentially strong upper body and neck)

Lordosis (exaggerated curve of lumbar vertebrae or "beer belly" look); anterior tilt of the pelvic girdle, short and tight erector spinae and iliopsoas, glutes, hams and rectus abdominis weak and elongated – helped by stretching erector spinae and iliopsoas, and strengthen glutes, hams and rectus abdominis (essentially strong "core")

Take clients through a full range of movement to ensure muscles are being worked to check posture and potential weaknesses
Ensure exercises are balanced across joints
Do a lot of mobility work (check out Bryce Lewis and Kelly Starrett on YouTube) and strength based exercises.

If you have scoliosis where the spine is twisted laterally (a sign is often shoulders at different levels) it requires medical intervention.

Go to your doctor and find out if there are any medicals requirements. Then go to your gym and ask one of the instructors. A good gym/instructor will help you out. There are also some useful videos on YouTube (a few by Elliot Hulse if I remember correctly).

Well I am working on my own postural issues.

First thing I would say is diagnose your issues:
http://www.exrx.net/Kinesiology/Posture.html

You may be able to do this yourself but probably need help. I ended up taking my own pictures and posting them here:
www.bodybuilding.com forum section called: Injury Recovery And Prevention

Then when you know what your issues are you look for ways to correct them. That forum has some useful posts on postural issues aswell.

I think one of the most important ways to correct bad posture is to simply be mindful of it and pull yourself out of it. That's not as easy as it sounds. Particularly this aspect I am having a hard time with. Also if you are in physical hurt pulling out of a slouch you may want to see a professional.

I am trying to strengthen the muscles that are weak and stretch the ones that are too strong/tight. But it all starts by knowing your particular issues and then looking for information on those conditions.

Ballet and yoga are my two favorites. I also have a back strap that I use while I'm not working out. The better posture I have, the better my bad back is. And I tend to hunch while I paint, so it really helps to keep my back straight.

By the way some lifting may make postural issues worse, such as potentially shoulder press and shrugs.

It's really important to know your postural issues. Some of your muscles will be weaker and some will be stronger pulling you into a slouch. You don't want to train the strong ones or your postural issues will get worse. You need to NOT train the tight/strong muscles pulling you into a slouch, instead stretch those (and foam roll). That way they get looser. Train the weak muscles that will help you pull out of a slouch.

Maybe get a trainer at a gym to help you get started.