My clothes ride in a gym/duffel bag that can be collapsed and zipped up into itself. I actually have two such bags; outbound, one is thusly compacted and stuffed in the other along with my clothes. If I've got a lot of souvenirs, conference take-homes or whatever, I'll expand it for the inbound trip and divide everything evenly between the two bags. Bags like this are pretty cheap, and it's easy to take one along all the time "just in case."