When travelling alone, I do packing the morning I'm leaving for the airport. That's only because I know exactly what I need ... since it's for work, it doesn't change appreciably.
When going with the family, and ESPECIALLY if it's a surprise, I start with a list of the activities planned. That, coupled with expected weather conditions, helps pare down the list of items to pack. The detail explained here is most useful when the kids are young ... my younger boys graduate from high school next year, and the oldest will graduate from college the year after, so it's not as helpful for me now! (and it's super hard to surprise them these days!)
After determining the expected weather and activities, I would consider the clothing that they all have that's the same, or nearly the same. Given that the parks are crowded, I dressed the boys alike so they were easier to find if they got away from us (and this DID happen on more than one occasion, I'm sorry to say!) I would pack, for each boy, a pair of shorts, shirt, undies, socks, and diapers (when they were very small) into one plastic grocery store bag, tie the handles together, and staple a paper tag through the holes with a description of the items within written on it. If my husband chose a particular racing shirt or whatever to wear on a day, often boys had similar matching shirts and for that day, the boys would be dressed like Daddy! When the clothing was to be used, I removed the tag and saved the bag. The dirty clothes would go in that same bag at the end of the day, was tied up, and ready to pack for the trip home.
One thing I forgot here ... I found it MUCH easier (learned from trips to daycare at 0500) to bathe the boys at night, and dress them from the next day before bed. Their clothing was only t-shirts and shorts without pockets, so there was nothing extra on it to annoy them as they slept. They woke already dressed for the day, and that saved LOTS of hassles (especially with one autistic boy).
Bagging everything, I found that I was not losing articles of clothing anymore AND this process served us well on the many roadtrips we would take!
d3boyz