Before Your Flight: Prioritize Sleep
Science has shown that sleep massively impacts your immune function. If you're missing zzz's, you're likely impairing your immune system and inviting in a nice hefty serving of rhinovirus, the common cold, or the flu.
If you can, get plenty of sleep in the week leading up to the trip, but at the very least prioritize a full night of rest the night before to prep your body for travel. Like training for a marathon, you're training your immune system to be as strong as possible in the face of influenza.