Whether traveling domestic or international, I always buy a bottle of water before getting on the plane.  (Not before security because it will be discarded due to regulations) but just prior to boarding.  If flying for 3-6 hours domestically, I like to have water ready to drink and not wait for the little cart to come around. This is an important step to staying hydrated and helps a lot with that nasty jetlag. Remember to H20 before you go!