As you can see from the responses, there are lots of ways to go about packing but not all (or even any) of them will necessarily be the best for you.
I'm an older guy (58 years) and I think I've finally got it figured out- for myself anyway.
I just got fed up with packing 26 inch suitcases with "stuff" and bringing home things I didn't even use. Now I just boil it down to what I need at Disney.
Most of the time that's shorts and t shirts. I really like the Disney white sport socks so I count on buying a few pairs while there and not packing as many to take down. I also figure I'll buy a couple of t shirts, so I buy those early on and wear them while there. And yes, I have even been known to take along worn out socks and underwear and just discard them as I used them.
In my 20 inch hard sided carry on I'll have 3 pairs of shorts, a few t shirts, half as many socks as I need, enough underwear, a decent pair of long pants (I wear jeans and a comfortable sport shirt, with a pocket for passport/ID, on the plane), one nicer button-up shirt, my best pair of walking shoes, wearing the more presentable ones, and basic toiletries except for liquids. In my under seat tote I have my electronics for in-flight entertainment, my documents, wallet, keys, phone, and liquids. Everything is in it's place for security screening from the start. My jacket (a fleece and windbreaker combo), gloves and hat go into the 20 inch once I'm in my home airport terminal and stay there for the trip. I also use packing cubes and roll my clothes.
I doesn't sound like much but it leaves room to bring back anything I could want and I never get caught with over-stuffed and overweight carry-ons at the gate. If we stay a week or longer, we do a few loads of laundry midway through our stay.
Hope this helps.