This prevents wrinkles and takes up the least room. This is a particularly effective method for travelers living out of their suitcases. You can rummage through your bag without messing things up. At least that's the theory. Take zippered plastic bags of different sizes.
Large ones are good for carrying wet clothes, for grouping underwear or socks, and so on.
If you carry a couple you'll find a use. Use stuff sacks of various sizes and colors. These are also useful for grouping similar items. A large net one is good for accumulating clothes that need laundering. Illegal drugs, anything you don't want searched by customs, illegal drugs, alcohol if you are under 21, illegal drugs, firearms, glass.