Using Dial soap, I drop the one almost used-up into the shower while I bathe, along with the brand new bar. Once they are softened, the used-up bar fits nicely into the indentation on the new bar. Just squish them together and once it dries, it is firmly attached to the new bar. I don't know if this works on other brands, but it definitely won't work on Ivory!
I've done that for years with any soap-- if you get the small bar soft enough you can stick it onto soaps that are any shape. I still think it's the best way to save on soap.
The second-best way is something we in Japan do all the time-- keep your soap hanging in a net from a hook. Use it through the net. I don't do this for large bar soaps (I think the net probably collects germs), but I sometimes put a few used slivers in a net and use it to wash the rough skin on my heels. You can use plastic net that fruit comes in.