I haven't tried this yet, but for my flight to the Caribbean this winter, I'm thinking about seriously layering clothes and wearing as much as I can -- with a string backpack folded up in my pocket so that I can strip top layers upon landing and stow in that bag! I wish I could wear the life jacket that I like to take for snorkeling, but I expect they wouldn't allow that.
Wear cargo shorts and stuff the pockets (with non-liquids). (I learned a long time ago to get along without a purse -- and I can even do a day at WDW with just what I can carry in cargo short pockets [now that small digital cameras exist]).
I do use a small to medium size backpack as my "purse/laptop bag" extra bag, along with the carry-on. When I take a trip that I expect to bring back more than I'm taking, I'll roll-up a duffle to check the extra stuff in (so just one-way for extra bag). If traveling with family, I still end up checking one bag for us, with extra stuff (but that's stuff that I wouldn't need at WDW).
I think you can use the roll-up tightly method without needing space bags -- I just use zip-locks and squeeze out the air. I like having stuff bagged because my suitcase often gets checked by TSA.
You're not going to see the same people again, so you can wear the same outfit more than once -- you don't need something different every day. And you only need one pair of walking shoes (that you will have on), and maybe slides/flip flops for the pool, for that short a trip.
Now, if you're doing the Deluxe DP and feel you need to dress up for nice TS dinners, that's a whole 'nother problem about packing.