Nesting doll method: Fill all empty spaces. Example: Stick socks and underwear in plastic grocery bags then shove those into shoes or corners of your bag. If packing an extra purse, fill that too. If you'll need a coat or jacket where you're going, wear it and load up the pockets.
Rolled clothes: Clothes actually come out surprisingly wrinkle-free, says a friend, who rolls up polo tops, sweaters, pants and even dresses, then stands all the rolls upright on end in a squishy gym-type bag. She claims she could pack enough for a month.
Layering: I've seen this done with clothes (on hangers) and it looks ingenious. You start by placing about half a pair of pants flat in a small suitcase with the other half outside the bag; then, place half of another pair on top again with half hanging out of the bag on the other side, and ultimately you fold the top parts over what's already in the bag but --maybe you'd better watch the video.
Human luggage: A company called Jaktogo offers some very unusual bags that convert into a coat or long vest or even a dress. In other words, wearable luggage, but I'll be frank: You will look strange. You will also get a whole lot of stuff onboard the plane and since you're wearing your luggage, you won't have to fight over bin space. Or you could just don a Scottevest and load that up. Or load up your own coat pockets.
The Guy method: I want to be perfectly clear, I have never tried this method but I've heard so many legends about it I felt compelled to pass it along. It's a classic lighten-your-load procedure that works like this:
A. Pack your oldest underwear (the ones with the most holes in them) B. Discard after wearing
I am told this method leaves lots of room for Vegas souvenirs.