This comes up from time to time and I feel like I’ve answered it in one or two of these (see links below). I use the extra bits of string I cut when changing string and the old ones that are being replaced as…string. Just goes in a compartment in the junk drawer and we use it to hang decorations, fix/make stuff for the kids or whatever other uses come up where we’d otherwise be looking for the kite string or something. And to be totally transparent, the compartments in our junk drawer are buried under more junk in said drawer. Organization is a skill I don’t try too hard on.