I know what you mean about not feeling that you are breathing deeply enough. Something I have learned in the last 10 years of studying meditation is to relax your levator-ani(Sp?)(anal-sphincter) muscle when you inhale, and constrict the muscle on exhalation, and this will give the sensation of breathing all the way down into the abdomen. Of course this also plays a part in martial application and body-tightening isometric-exercises such as Sanchin-kata.
Hope this helps a little,