How do you pack for such a trip?

One of the more daunting parts of planning a trip around the world with a pair of 12-year olds is how do you pack for a trip of such a duration?  Back in 2009, I took a trip to NYC to meet a friend to see the U.S. Open at Bethpage Black, play some golf with him on Long Island as well as meet my cousin from Connecticut in Manhattan.  Knowing I didn't want to take golf clubs on the train or in a taxi into Manhattan, I decided I would rent clubs and just put everything I needed in a backpack suitcase, which worked great.  That carried over to our trip to Europe last fall, and is the blueprint for the trip we are about to take.