I usually keep a "scraps" container for my odds & ends of craft supplies (ribbons, paper, yarn, fabrics, scrapbooking scraps, etc.) and then let the kids use that instead of using my "good" stuff.
As far as soap is concerned, if it's bar soap, I'll try to adhere a new bar to the old one, while the old one is still soft and wet. Hand soap in pump bottles, I usually water down a little to get the last little bit of the soap out, or I'll take the pump/lid off the bottle, and dump the leftover into a newer bottle. Same goes for dish soap (Dawn).