My mother always made her own soup, and she would keep a plastic container with a cover in the freezer to toss leftover veggies into to use.
I use bars of soap until it is just a sliver and then when the new bar is just a little past new, after a shower when both are dampened, I meld them together to use up the sliver. It's kind of fun if they are different colors.
When I brush my teeth, I use about 1/2 an inch of toothpaste. It's still more than enough, and doesn't waste the rest that would just go down the drain.