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Try
Airborne. A lot of people swear by it. I've taken it a few times and haven't caught a cold after flying those times, fwiw.
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This stuff is the greatest thing since sliced bread. Though it's really expensive (it's like 9 bucks for ten tablets, take 1 tablet every half hour you are around sick people or in a crowded plane). When I was in airports/planes for 24 hours I took one at the beginning of the flight and one at the end and didn't get sick *shrug*.
The other thing I often do is ingest lots of Vitamin C and water before flying. This can either be done in the form of orange juice or vitamin C tablets and water.
My parents are crazy and wear doctor's masks. They say it helps a lot (until they started doing this my mom would invariably get sick every single time she got on a plane) but... they're wearing doctor's masks.