I made a master packing list for our last trip (based on some "ultimate packing lists" I found online), which I pared down ruthlessly as our trip got nearer. It worked beautifully, so I'll be using the same one this time around.
We are light packers, taking only 1 carryon bag and 1 "personal item" (purse or backpack) apiece. We've found that a small, 21" carryon (per person) has plenty of room for our clothes for a week (for me: shirts for each day, shorts to be worn twice each, one swimsuit, one sundress that doubles as a swimsuit cover-up, 1 pair sandals, socks and undies, all organized in mesh packing "cubes"), plus a few toiletries and sunblock, meds, miscellaneous items (e.g., wet wipes, clothespins, a nightlight, empty laundry bag, cell phone chargers) and breakfast items, since we eat breakfast in our room (mini cereal boxes, granola bars, dry flavored coffee creamer, disposable bowls and plastic spoons -- we buy milk at the resort). We wear our bulkiest clothes to travel (sneakers, pants, a fleece jacket) and bring other miscellaneous items (cameras, extra batteries, books or toys, for the kids etc.) in our "personal items." Honestly, that is all you need. If you find that you just don't have enough clothes, laundry facilities and supplies (and even your hotel sink - I'm told that the Disney shampoo works nicely as a light detergent, in a pinch) are readily available if you need them. As a previous poster said, you don't need to worry if you forget something: there is virtually nothing you can't borrow or buy once you get to Disney World.
For me, a good list takes all the stress out of vacation planning. Once I'm satisfied that the list covers all the essentials, I can rest easy, knowing that once all those items are packed and checked off, I cannot have forgotten anything important.