When you think of Ziploc bags, you may think of the items you put homemade sandwiches into. But a friend introduced me to Ziploc Big Bags, which hold three gallons each and are my version of packing cubes.
Not only do they make you a more organized packer, since you can color-code what you're packing and see everything through the see-through bags, but you can then squeeze out the excess air once you're about to seal them shut. Plus, you can save a couple empty ones for dirty clothes.
Added bonus: Bed bugs are sometimes an inherent part of traveling and they hate plastic, so if you have all your items sealed in these Ziploc bags, it's yet another bed bug precaution you can take.
For smaller items, I also use Ziploc's gallon-sized freezer bags, quart-sized freezer bags, and sandwich bags. They also make great portable laundry machines — just put in a clothing item, some liquid soap or shampoo, and water, then close and shake it up, and voilÀ!
I bring extras of all four types of bags, plus a couple of garbage bags (in case there's no bathtub or shower to store your bags in, a preventative bed-bug measure), as you never know when you'll need them.