I haven't tried this at WDW but at some other hotels, I'll put my empty suitcase under the bed to save room. Some people like to put their dirty clothes in their empty suitcase until they leave, which saves having the dirty clothes out in a hamper.
We do that on cruises; with limited closet space, it is easier to just place the dirty clothes in the suitcases in the closet, rather than using the pop-up hampers.
Anyway, my suggestions have already been mentioned....bring wire hangers and leave them behind. I also bring a ziploc for ALL of those receipts. Seriously, how do we manage to accumulate so many?!?!? Have a great upcoming trip! 
As a PP mentioned they are great for keeping kids organized. I put DD's outfits in individual bags so she knows to just take out a bag and everything she needs is in there - even hairclips and socks. If you squeeze all the air out of the bags they are super slim, like those storage bags.

