ONE. Drink a head turning amount of water.
Dehydration creeps up on you while traveling. Sitting in a seat for hours, sweating ever so slightly, humidifying very dry air with your breath, battling inflammation and fatigue, - your fluid demands are up there. But, are you drinking more? A LOT more? You should be. My simple rule for travel is to drink until your pee runs clear. This will ensure your body is getting all the fluids it needs to flush, cool, circulate, and rehydrate. Yes, I recommend the aisle seat!
TWO. Wash hands OFTEN.
I am not a 'germaphobe' and do not use hand sanitizers usually, but I do think there is a time and a place for hand cleaners and keeping hands exceptionally clean. Many folks complain of the lack of bacteria in our digestive tracts and argue against hand cleaners on that basis. However, I think the argument falls flat, especially when travelling - it is not a shortage of bacteria, but a shortage of GOOD bacteria that bites us. When travelling, especially in a plane, you are sitting in a human made test tube of everyone's bugs. Add in some sleep deprivation, some stress, a bit of traveler's diarrhea, and a likelihood of eating nutrient deficient food (and typically gut damaging) while on the road, and it becomes a case of "give me your bugs and I will grow them for you". To avoid this, wash your hands often, be very careful of what you touch with your hands and what you put in your mouth (skip the hand snacks unless you have just washed your hands) and on top of washing your hands often, feel free to be a snob with your hand cleaner and to decline anyone's hands (including yours) touching your food/face/mouth.
THREE. Don't eat gut damaging or pro inflammatory foods.
If it contains gluten, skip it. Even if it is not gluten-laden, if the food is crap, just SKIP the meal. Pre and during travel is not the time for that cheat meal, or to start your "holiday binge" or to inflame your gut, or increase gut permeability to allow gram negative bacteria (the bad ones!) into your system more readily. Did I mention that you are locking yourself in a human test tube, full of bad bacteria and not so friendly viruses? Don't give them a head start - give YOURSELF the head start - keep your gut lining healthy and intact!
Specifically skip the gluten, the sugary cereals, and any dairy you don't normally consume and tolerate very well. Avoid the heavily processed crap they try to feed you before, during, and after your journey. Better options are to take your own food, or be very judicious in your eating selection, or just SKIP the food on offer, and fast your way between safe eating destinations. Make it as hard as possible for any ingested nasties to get into your system though a damaged gut lining, while keeping inflammation low and your gut healthy so your immune system has the bandwidth to smack the nasties that do get in.
FOUR. Keep a Clean (Sinus) House.
Perform a nasal rinse beginning two days before, during, and for two days after your flight. If you consider the same principle as the one above of eating cleanly to maintain gut integrity - if you are going to be more heavily exposed to things that want to hurt you - make it as hard for them to set up shop as possible. In this case, we want to leave your sinuses as unblocked as we can, while flushing out any rapidly growing microbe colonies. This one works a treat, and is doubly important if you intend on doing any scuba diving at your end destination. Free diving in the salt water of the ocean by the way, is a great way to score a solid nasal rinse for free while looking at fishies. You can get these at your local pharmacy or online.
FIVE. BE Proactive with Your Nutritional Insurance.
This part is the easy part. To stack the deck in my favor I just load up with an "immune" daily dose of Athletic Greens from three days prior to the trip through to two or three day's after my flight or drive. For me that means taking 2-3 servings each day for that period. I also make sure to double up on my daily probiotic about 1 week prior to and during travel. This will almost always prevent any occurrence of traveler's diarrhea, I rarely get so much as a sniffle, and this leaves my energy rocking super high when I get to my destination - which is perfect, since it allows me to do what I traveled to do... enjoy my holiday!
I hope you do the same (enjoy the holiday) and incorporate these tricks to make this happen.