Probably there are some genetic reasons involved in your case, as well as lifestyle.
Does your mum, aunt, sister etc. have puffy ankles, varicose veins etc? Usually men do not suffer from it, due to us having more testosterone and less estrogen.
Even if that's the case, all is not lost! Lifestyle can improve or worsen genetic traits, so you still have control.
The contraceptive pill, constipation, other medications, lack of antioxidants (berry fruits, veg, spices, herbs), lack of protein in diet (fish, poultry, meat, beans, eggs), sugar, too much caffeine, alcohol, inactivity all cause water retention - these are the things to focus on. Just to let you know that it is not just what you are currently doing, you may have made poor lifestyle choices in the past for a few years and that weakened your blood/lymph vessels. Age usually does not cause problems by itself, the cumulative effect of years of poor lifestyle does.
I suggest you make the above lifestyle changes, eat more oily fish (especially salmon) and also use horse chestnut, red vine leaf and butcher's broom extracts, either in supplement or cream form. Creams rich in essential oils of mandarin, juniper, cypress, rosemary mint and caffeine also help (caffeine locally is beneficial, only if taken internally causes problems)! Our Lipo Actives cream contains most of the above ingredients - in high concentrations. You can check it out at http://www.lipotherapeia.com/actives
Keeping your legs elevated when you lie down is also helpful, as are lymphatic stimulation massages and pressotherapy.
You can also do some simple calf exercises before, during and after flying. The gastrocnemius muscle is called the "second heart" for it's importance in helping to pump blood and lymph onto the upper body from the ankles and calves.
Self massage before, during and after the flight is not a bad idea either.
And keep wearing those flight socks - they still help a bit.
Hope that has been of help!