My extremely frugal farming grandparents taught me this one: get a cheap facewasher from the cheapest shop in town and then fold it in half, hand stitch one end and the side together, and then use whatever you have handy to put a resealeable close along the other end. (I had some old press studs left over from a sewing project that I used, but you can use hooks and eyes, buttons, drawstrings, whatever you like). They would then put the whole bar of soap in the folded over facewasher and then use that in the bath. When the bar of soap started getting really small, they would just open the top and stick the next bar in and close it again. You have to make sure to wring as much water out of the washer when you are finished as you can and to hang it up so it can air dry as much as possible, because if you leave it too wet the bar of soap inside goes mushy really fast and only lasts a couple of days. But if you do it right, you have a nice soapy washcloth to use and none of the soap ends ever get wasted. It also means you don't drop the soap in the bath or shower and have to go after it, only for it to slide everywhere.