I saw this one somewhere a while back. It is as simple as setting out the clothes your kids need for each day on the trip, including underwear & socks. Bust out your ziplock bags (or if you are super cool- sew some cute drawstring bags- one day, I will work on that!)
Stuff each day's clothing into a ziplock, label as needed, and viola! You know what is available, all the parts are there, they will stay clean and dry, and you have a new bag to put the day's dirty laundry in. I love it! Plus, it keeps the luggage organized.