Here are a couple of suggestions from "road warriors" that I have found work for me:

if you get colds/flu-like symptoms after you fly, spray Afrin or a similar spray in your nose before you board the plane - and during a flight over a couple of hours.

if you get sinus type infections every time you fly (like I used to, on every flight over 3 hours) take Sudafed before you fly and shoot Ayr or another saline solution nose spray in your nose a few times during the flight.

And, if you can find a French homeopathic remedy called L52 (if you know anyone traveling to a French colony, or France) - it really works! Not made with Osillicocsinum (or however you spell it)... If you take it as soon as you feel yourself coming down with a cold or flu - it goes away! Really!