How do you pack for a 21 day vacation?

In about 18 days we will be headed for the hot and dry desert of sunny, hot, sweaty Arizona. I am really looking forward to being home for the hottest time of the year, so I can remember why I tolerate -10 degrees in the winter. No really, I'm looking forward to seeing the sites of the desert, the smiling faces of family, and the wonderful food (not to say we don't eat here in Montana, but it's not the same). So what I'm not looking forward to is packing for 6. We have never flown as family anywhere, so that is a whole different kind of packing. Hopefully my mother in laws washer is in good working order, because I'm sure we'll be doing plenty of laundry. At least shorts and t shirts take up a lot less room in the bags than pants. And we'll be packing a lot of those. Okay so any tips for airline packing???? Does anyone know any tricks I'm not aware of???