For beginners (which you aren’t, but there are others reading…): a whole lot of reading up on anatomy, over and over, and even a bit of HippyDrome’s site and ones like it, that talk about where to place the collar bone and how the collarbone and shoulder are placed.
Then, I just use a Rigify rig currently. Place the metarig like so… generate rig, parent her mesh to armature with automatic weights, put her in extreme poses, see how it’s really bad, think carefully what that means, go back and tweak the metarig, repeat.
After some time I realised I still wasn’t getting good shoulder or butt deforms. Again, literally staring at her in this pose for several hours and thinking about how muscles and skeleton fit together, it hit on me. Rigify is missing a couple bones. There is the scapula that helps hold the structure of the upper back near the shoulder, and the gluteus maximus, which similarly in the butt holds shape when the leg is moving.
So I added scapula and butt bones to the metarig, regenerated the rig, and this is what you get. I have highlighted the collarbone (shoulder), scapula, and butt bones (right side only) for this pose so you can see them holding the shape. Again, the collarbone is included in default Rigify metarig, the other two are new.
I also, after generating the rig, would originally try copying the rotation of the collarbone with the scapula a little bit, but I found that 99% of the time, just having the scapula there holding the shape, it’s enough, and I will manually rotate or pull it when I need to. I also toyed for a while having it do a stretch-to constraint to her arm, which also I’ve found not needed most of the time.
The butt bone literally never moves. It seems just having a static bone there is enough, which means I probably could have weight painted this problem.
There is no weight painting or shape keys at all. This is 80% armature deform, with some vertex groups at her shoulders, knees, neck, etc that then have a corrective smooth applied. Then there’s a general (very light-acting) general corrective smooth over her entire body. Those corrective smooths are about the other 20% of the effect.
I’m not sure I’d recommend this workflow - anytime you parent another object to her armature, you have to copy all those corrective smooths somehow, or that object won’t deform the same as her body. EDIT: I guess if you use shape keys, you also need to figure out how to apply that to the geometry above it - so maybe this workflow is superior, if it can be made to work?
In the render, please ignore the fishnets problems - this is a very early test that I have deemed a failure, and it won’t survive the night.