Eagle Creek Pack It systems, I know it's already been mentioned, but it's worth saying again.

I'm also starting to become a fan of those roll up bags that take out all the air and compress your stuff, but usually only do that on the way home because of the wrinkle factor (don't care if dirty clothing wrinkles or not).

Stuff your shoes with your underware and socks, wrap breakables in clothing instead of takes up too much space bubble wrap, Wear the shoes that take up the most space, get travel sizes of all the stuff you are carrying, or save your near empty toothpaste, shaving cream, etc and just use them for the trip and toss them at the hotel, no sende in carrying a full can of something across the ocean just to use it for a few days and carry home again.

Tossing stuff also gives you room for the stuff you purchase.