Carry a Portable Battery Charger
Airports have charging stations, and that's usually the spot everyone gravitates toward. Often you won't be able to get a seat next to a socket, so you end up standing off to the side while keeping an eye on your phone that's charging. To be on the safe side, carry around a small portable charger that you can pull out anytime.
Remember to put your phone into airplane mode once on the flight. It's not just for flight safety: if you accidentally leave it on as normal, your phone will spend the whole flight scanning for a network, and the battery will be drained by the time you land.