Ok, let’s not make this sound more scientific than it really is. The trick to staying hydrated whilst flying, or in any situation really, is to drink water… lots of it and on a regular basis. But this is not what I mean.
Most of my flights are short haul, so I don’t have to worry too much about dehydrating really.
But even if my flights are rarely longer than one or two hours, I need some water in between, so I almost always end up buying a bottle at water at the airport… after security obviously, so that the 100ml fluid rule does not apply.