We go a lot (2x a year), try to only take 2 large suitcases (4 of us), stay DVC so have access to laundry, rent a car, and never tell the kids in advance.
I usually start gathering up things like laundry packets, dishwasher tabs, mickey ears, sunscreen, etc about a week before. The actual packing I do the night before we leave after the kids go to bed as I am finishing up laundry (since I am usually pulling things out of there as I pack). This last trip in May, I was working INSANE hours at work. We were staying over at the airport since we had a 6am flight. I had NOTHING packed ahead of time. I had plans of leaving work around lunch, but didn't get home til 4:30. I had myself and the kids packed (DH had laid out all his stuff earlier) and we were out the door by 6pm and that included prepping the house for us to be gone for a week. I have it down to a science....although warm weather trips are much easier to pack for than trips during the winter when you need a little bit of everything.