Carson’s Stuff-It is a microfiber lens cloth tethered to a small pouch. Pull the cloth out of its pouch to use it, and then stuff it back in for safe and clean storage.
The Stuff-it also has a clip, allowing it to hang off your gearbag always at the ready for quick access. I’m the type that breaks off zippers and clips, so I keep a Stuff-It inside my camera bag instead.
You can find these for about $4-6 from a number of electronics and photography equipment stores.