Encourage your kids to think of mix-and-match outfits for various activities, just as they do when dressing paper dolls. (You even can have them practice by packing a doll wardrobe trying out the different outfits while they pack for themselves.)
For example, ask a preschooler, "We're going hiking. Which of your comfortable pants do you want to wear?" After he lays these out, ask him to match them with two T-shirts (for two outfits), a sweatshirt in case it is cold, and appropriate shoes. Then, consider another vacation activity. Ask him to find two bathing suits, for instance, with a sun cover-up and a hat. Next, ask him to think about nighttime, laying out toothbrush and toothpaste, pajamas, a beloved but small stuffed animal, bathrobe, and slippers.