Stay hydrated. Drink water.
I tell people this all the time, including–and especially–for plane travel. Why? Because the air in planes is not only stagnant and stuffy, it is incredibly dry.
In-flight air is drier than any of the world’s deserts. Relative humidity is 20-25% in the Sahara or Arabian deserts, while optimum comfort is around 50% humidity.
In-flight cabin humidities gradually fall on long-distance, high-altitude flights; in may cases approaching 1%. source
And, according to this great tip from Ideal Bite, you can save a little bit of CO2 emissions, by “going” before you go.
Bon (healthy and sustainable) Voyage.