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Can work on some of the rib cage musculature (which includes some of the serratus anterior , and external obliques  in the front and  posterior serratus in the back) by fixing your rib at one point and then gradually rolling behind the fixed point and then gradually in front of that fixed point. As you move away from the fix point try to move your sternum/spine away to lengthen the muscles. This teaches the muscular sling around the fix point to gradually lengthen as you are generating these movements. You can also do this in a vertical position with your ribs against a firm chair/seat/wall. It's like trying to roll up a carpet/sleeping bag and then gradually lengthening it while unrolling. 

Alternative: using the arm opposite the side you want to activate, place your fingers in the orientation of your ribs. Brace your spine by pressurizing and activating the abdominal cylinder enough that when you move your ribs that you don't rotate. At first pull the ribs away from the shoulder blade with your fingers. Then using the rib muscles pull the ribs up towards your shoulder blade along the line of the ribs. Then gradually uncoil and move the ribs away from the shoulder blade while keeping the muscles active. Explore different sections of the rib cage. You can also place your hand on the shoulder blade (may need to use a firm surface instead of your hand to hold on the bottom of the shoulder blade). In a diagonal line (following the muscle lines) draw you shoulder blade towards and then gradually away form the spine.