During the Great Depression and the war that followed it, soap was hard to come by. People came up with all sorts of tricks to stretch what little they had. My grandmother used to open the box of soap on one end and allow the bar to 'dry out' for a few weeks before using it. This removes the excess moisture content from the soap and makes the bar last longer. I haven't performed any scientific experiments to see if this is true, but from my personal observations I believe it to be.
So get going, open those boxes and use less soap.