In the days when people used bars of soap, we always stuck the last sliver of the old bar onto the new one.
In fact, Pears soap (anyone remember that, it was transparent) actually ENCOURAGED it, explaining that was what the hollow in the soap was for.
I never dilute things like shower gel to get the dregs out, instead I put a funnel into the new bottle then turn the old one upside down into the funnel. Sometimes it takes hours to drain out, but eventually there is no waste at all.
And I cut open toothpaste tubes and can get a few days use out of what is stuck inside.
Also, when I boil a kettle, I put any surplus water into a vacuum flask, which keeps hot ebough for most uses, or I can pour it back into the ketle to reheat, it stays hot for about 10 hours or more and saves a lot of electricity. I don't heat up more than necessary, but you have to fill it to the minimum level, which is more than a mug full for making tea etc.