I swear by the zip lock bags. I have some of the space savers that I got as a "gift" from my credit card company. I use them for blankets and big things, but for packing, the zip locks work just as well. I can't tell you how nice it is to just hand the kids a bag, and everything is all coordinated. One thing to watch out for is weight though. Once the air is squeezed out, you can pack a lot more into a suitcase, and it might be over the airlines limit (not to mention the limit of your arm!)
The first time I tried to use the ziplocks, I couldn't figure out how to get the air out. I zipped it most of the way closed, stuck a straw in the hole and sucked out the air. It was the funniest thing!!! Finally, I figured out all you have to do it sit on them, and that works a lot better!