Overpack snacks, underpack toys. Kids get crankiest when they don't have familiar things to eat. Also, food can double as toys; make mosaics out of colored Cheerios, for instance. And kids will play with anything (cups, napkins, sugar packets) and will also accumulate toys (from fast-food meals and souvenir stands) during the trip, so don't take the whole toy bin along.
Put extra clothes in your carry-on -- your baby may have a big diaper blowout on the plane.
Pack each day's outfits in a one-gallon Ziploc bag: shirt/pants/socks. It makes packing easier because you can keep track of how many days of clothes you have; after an outfit is worn, use the bag for yucky laundry or dirty diapers.
Even with all those clothes, count on doing laundry while away. Dirty baby clothes stink.