I usually have an extra either packing cube or plastic bag, depending on what it is (socks and underwear for example go into a bag). It starts the trip empty, and slowly gets full of clothing that is for the wash. By the time it's full, another cube or bag is empty and ready to take more stuff.

If I have socks that are ready for the trash, or a tee shirt that I'll wear around the hotel that is ready to die (working for non profits, you have a never ending supply of event t-shirts), I'll bring those on a trip and toss them instead of bring them home to throw them out.