I am a big believer in traveling light and that means that everything that gets packed has to be able to do double duty, be lightweight, perhaps be be suitable for another use and dry super fast. SHOOin Expandable Travel Shoe Bags for Carry-on Luggage is one such item that I like to pack.
The company provides two nylon-spandex bags, that are black and come with a drawstring and cord lock to keep bags closed. Spandex is lightweight and what I like about these black bags is that they stretch. I can put two flats or even bulky tennis shoes in them. The bags can do double duty as dirty laundry bags or just bags to hold stuff in. Since spandex dries super quick, the bags can be washed at any time during a trip rest assured that they will be dry in a day.
When the shoe bags are used for shoes, they protect clothes from the dirt left behind on shoes. The drawstring has a closure gizmo on it, so you can pull the cord to close and lock the opening. I really like my shoes bags and won't leave home without them.