Dehydration can be avoided by drinking fluids throughout the flight.
Make sure you carry minimum items that you will need during your flight.
The dry air and high pressure of an airplane can cause dehydration and irritation, therefore if you use contact lenses, it's a better idea to swap it for glasses.
Apply a high quality thick moisturiser to ensure your skin doesn't dry out.
Exercise your lower legs and calf muscle, so during the flight to keep the blood flowing more freely into your legs. This encourages blood flow.
Loosen clothing, remove jacket and avoid anything pressing against your body.