Here are two very different approaches:
Through movement, through thinking about it and imagining it, try to soften your sternum and let it melt inwards. Your arms are long and alive and their power comes directly from the spine and goes through the sternum. Move and let your arms lead you into space. Do some body work together with a partner: help each other to feel your thorax changing as you breathe, moving up and down and in and out.
The other approach is: Lift your arms. Point your fingertips to the ceiling or the sky. Now stay there. Stay there very long until you start to worry. Then take the arms down, not completely but until they are parallel to the floor. Now stay there. Lengthen your arms more than you think you can and feel the sternum softening. Stay there more and more, until you can’t any more and then stay some more.
When you finally take your arms down you will feel different than you did before.