When sitting, I strongly suggest that you try to relax your diaphragm so that you breathe through your belly rather than have your chest expand. In other words, let the lungs drop down instead of pushing out your chest. This takes a little practice to get the hang of, so try it, but if you have trouble with it, don't worry. Do it a little each time you sit and it soon will become easy.
It is very relaxing and not constricting. It allows for fuller expansion and penetration of the breath. You might have to make a conscious effort to push out your belly and find it awkward to do so. Eventually it will become a natural method you will do all the time. Just try to catch yourself whenever you find yourself expanding your chest with the breath. Relax, and let the lungs expand down.