I give each child a packing list.
Their backpack list will say things like:
2 books
1 small stuffed animal
3 small toys
1 water bottle
(etc)
Their suitcase packing list will include:
2 pairs summer pajamas
8 pairs underpants
2 bathing suits
(etc)
They are then given a laundry basket to put everything in.
We then go through the packing list, making sure that everything is there, and that they haven't brought (much) that isn't on the list.
I may then afterwards adjust on my own, if needed.
I like this because it gives them the structure to successfully pack themselves.
The idea is that as they get a little older, they can then make their own packing list, I will check that, and then they will pack themselves.
(My oldest is 9, so we aren't there yet.)
We drive 8 hours, so I have to allow mine to bring several things to do on the road. However, they always bring too much and it makes me batty! I love this idea of giving them a list! It will give them some ownership, but keep things reasonable. Borrowing this tip for when we leave Friday!