Nutrition: Before travelling avoid salty, fatty food which can lead to intestinal discomfort, especially at 30,000 feet. If you feel your body could use a boost, some people swear by multi vitamins which you can before and during your trip to make sure you're not lacking any essential nutrients. Hey, even if it's a placebo your immune system may still be convinced! If you feel like you might be coming down with something, be sure to take a day in bed and try to tackle those symptoms before you get on a plane – the travel will only make things worse.
Hydration: We can't say it enough – water is your best friend! The humidity level on an airplane is lower than the Sahara, so drink up. You may feel like warming up when it's chilly out, but the caffeine in coffee, some soft drinks and tea will only dehydrate you. Same for the alcohol on the plane – sorry! Stick to water or juice so that the only thing you have to deal with when you land is finding your luggage.
Stay active: Sure, the simple act of keeping warm when it's below zero out can feel like you're burning plenty of calories, but your immune system needs you to actually exercise if it's going to continue to function at full power. Walk around the airport instead of sitting at your gate, and while on the plane, try to take a stroll up and down the aisle and do some gentle stretching in your seat. If your hotel or furnished apartment has access to fitness facilities, use them – the exercise not only relieves stiffness from travel, increases blood flow to muscles and heart, and helps ward off the blues, it can also help beat the symptoms of jet lag.