All my kids are skinny little thangs. In fact, my youngest perpetually hikes up his pants even when they are not falling down. He's just used to his pants slowly creeping down. My daughter has the same problem and not all jeans have the wonder that is the adjustable waist. To solve the dilemma, I made her a super quick belt from a piece of vintage trim I found at the thrift store. I hemmed the end, sewed on some D rings and voila! The pants stay on! This technique would also work for an adult, but I think it would be best to stabilize it with a webbing or fabric base.