To add a bit more info to the extra fuel tanks that were pictured in the earlier posts, after the aircraft gets to Hawaii or where ever, the tanks are shipped back to Boeing by ship (cheaper).

If you take a look at the floor of a plane, there are tracks that the pax seats snap in and out of. It's not that hard to snap out a few rows of seats, ship them by ship if you need to keep the weight down in the plane, and snap a few extra fuel tanks in place.