I never pack too heavy (too much formula, etc.) when traveling with Armelle if her favorite formula, food, etc. is available at our destination, since she is handful all by herself. I do take the necessary emergency medicine along (with a letter from a pediatrician to allow me to carry it on board), including a few small bottles of clear Pedialite.
Make sure babies are sucking/drinking during take off and landing as this eases the pressure for them. When Armelle was smaller the pediatrician also advised me to put a small drop of the saline solution in her nose to keep the passages clear and again ease the pressure.