Lots of hydration in the days leading up to the flight. I try to walk the thin line between not enough water and too much, which causes too many trips to the often horrible plane bathroom. Hydration the two days before the flight works best for me. The worst is realizing on flight day that you haven't hydrated and you're an ounce of liquid away from having your face crack open. Then you hurry up and drink and wind up wearing a path between your seat and the bathroom! Also, hydrating on an empty stomach is not efficient, as the liquid tends to run through before it can be absorbed.