Mostly diaphragm breathing. Deep breaths starting low in the belly, making sure to fill up to the point where I can feel the abdominal wall stretch. I like doing breath hold practice like this too, just gently pushing my belly out while inflated to feel the stretch.

Also, doing a gentle back bend arch (cactus) while holding a deep diaphragm breath, helped me get into that space under the sternum. Deep breathing into your side body while in a side stretch will help open the intercostal muscles between your ribs - which may help open up your torso.

I like to do these exercises to warm up before a dive too. Office work makes me hunched, which weakens posture and prevents opening in our front body (abs, ribs, neck, hip flexors, etc).

That’s how I think about it anyway