Keep the little pieces of bath soap. When you get enough, put them in a small pot and melt them on low heat. Use aluminum foil as a mold. when the soap is melted, pour into the mold. When it hardens, peel off the mold. It is never-ending but still. It saves on bath soap.
Or just stick them in a small net bag and use. You use them up and you get a scrubby.
Wow, that's a lot of work and uses a fair amount of energy (heating the soap, hot water for washing the pot, etc.).
When the soap gets down to a sliver but before it's small enough to break, get a new bar of soap and just do the Vulcan soap-meld by holding the sliver onto the new bar and use. After a few wetting/drying cycles the sliver will be one with the new bar.