I thought I was a light packer until I met DH. I swear the man could pack for a yearlong excursion in a single carryon. The most important thing he taught me was that just because I use it/wear it on vacation doesn't mean I need to pack it. So that means, fewer clothes (and yes, we will do laundry on our 6 day trip), fewer books, and fewer shoes. I only take a pair of
Crocs to walk around the parks in and go to the pool, and a nice pair of sandals for evening meals. Since I have to wear something traveling to and from WDW, this means packing only 1 pair of footwear. Of course it helps that I have tiny feet

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We fly, so I like to wear my heavy stuff and pack my lighter things. This also means I only bring along 1 pair of pants. As others have said, stick to clothes that mix-and-match such as neutral or solid colors. One tip that I found helps is to bring one nice outfit and a scarf or two. I have a dress that looks totally different and cool by just adding a scarf. Two outfits in one! The scarf takes up almost no space especially compared to a second dress or skirt/top combo.
To lighten my load even further, I don't bring any toiletries except contact solution and toothpaste/toothbrush. I can live a week with Disney care products.
Finally, keep in mind what can easily be shared by a family vs. what each individual should bring himself. For instance, we only pack 1 camera, 1 laptop, 1 phone charger, 1 tube of toothpaste, and 2 books that we share.
My goal is for my bag (carryon size) to be no more than 1/2 to 2/3 full when I pack it because I always find something to take home with me. And I'm the one carrying it through the airport, so I have no one to blame but myself if I've packed too much.