We've been travelling with our girls since they were infants. As they grew older, they asked to help. We'd tell them which clothes they needed and they'd get out stuff. Sometimes we'd put stuff back when they were sleeping and get out what they needed.
As they got older, we started giving them lists. We'd tell them how many outfits they needed and provide some direction about short or long sleeves and pants or shorts. They'd pull things together in outfits and we'd sort through things to make sure they fit/matched/etc.
Now we just give the girls lists and let them pack. We don't even look at what is going into the suitcase. We tell them to get their toiletries and they know what they need.
Once in a while we end up with a surprise, like the brunette twin's two pairs of jeans for our current Florida trip or the blond twin's never-ending supply of hair stuff. Most of the time, though, they do a great job with their clothes and stuff to keep them busy as we travel.
This was the plan, of course. We wanted them to be independent and responsible. There were times when they were younger that we didn't think we'd ever get here, but now that we are it makes vacations a whole lot easier.