I pack enough 'bottoms' for half the trip in neutral colors, like khaki or in jean material. (when it's not hot and sweaty I wear them twice, if it's hot then I plan for laundry)
I take one shirt for everyday, then put in two extra either incase of spills, sweat, etc. All shirts must go with any pair of bottoms I decide to wear. I never take clothes that only go with certain pieces. Mix and match is the way to go.
Underwear and socks one for each day AND at least 2 pairs extra, sometimes 3. (these NEVER get worn twice

)
I take one pair of jeans or crops and one sweat shirt/hoodie 'just in case'. Often the jeans or crops are my plane clothes as they are the bulkiest and take up more room in the suitcase.
One pair of sneakers (two if I have room), one pair of flip flops. I've never needed dress shoes at Disney.
I stick to a basic any piece can go with any others, and I try to include at least one piece in each that can be dressed up just enough to wear to an ADR like HBD or Yachtsman.
Watch and maybe one pair of basic stud earrings.
Basic toiletries with as many cross purpose products as possible. (my shower gel doubles as shave lotion, my hand lotion can also be used as body lotion, etc) I dislike the Disney line of hair and body products, as well as having skin allergies, so I don't ever plan on using them. And usually a small kit of nail clippers/file/eyeglass repair kit/first aid.
I don't wear make up on vacation so only the minimum needed (tinted moisturizer, mascara, colored lip stick).
One small bag for parks.
Usually a small stash of Starbucks Via packs/snacks.
All of this fits in one 26' checked suitcase
In my backpack as carry on:
Cpap (sleep apena machine)
I use a tablet so no books, mp3 player, dvd/cds needed every thing on this.
Phone also doubles as camera.
Spare glasses/sunglasses
Small baggie with hand sanitizer, chap stick.
Charger/headphones/etc
Snack for plane (I don't do honey roasted peanuts, yuck)
Honestly I feel like I over pack.
My one January trip I took crops only and wished I had shorts, only used a hoodie, never needed an actual coat.