Although I can make my BJD with a one piece torso, creating joints will increase her poseability. There are a few things to decide when considering torso joints. I have to decide how many joints I want and where to locate those joints.
There is a good drawing of a few ideas about how to do the torso joint on the Enchanted Forums.
At first I was thinking of only having one torso joint, but ended up deciding with two joints to help increase poseability while hopefully maintaining an aesthetically pleasing natural look. Her body is meant to be child-like so I don't expect her to bend like a gymnast.
Above you can see my drawing of how I would like her torso joint to be. The first joint will be at the bottom of her rib cage and the second joint will be at her pelvis.
There is just one main problem with making perfectly round ball joints for the torso. When one looks at the top view of a person, you will see that a person is longer side to side than front to back. So the torso would be more elliptical than a circle. You can see my illustration of this on the bottom part of the drawing. This is one reason why I decided to do 2 joints instead of one. Each joint will have a lesser degree of twist so that it will reduce gaps and keep parts from sticking out oddly.
So far I have only created one torso joint which is looking pretty good so far. I will be adding the second joint soon for the lower torso.
Something else that needs to be considered when designing the torso is how I plan to make the silicone mold for it. Will it be easy to make a mold or or complicated because of how I made the joints? That is on my mind as I design the torso joints.
And last but not least, I still have to consider where to put stops so that the body is not posed in ways it is not meant to be, thus causing odd angles and pop outs.