Don't use coffee. Drink water instead to keep yourself hydrated. Use a downer like melatonin or 5HTP and sleep. When you wake up you're refreshed, so make sure you land in morning. Or at least not in evening while you slept in airplane.

Clothe yourself warm; sweater, good coat, hand shoes, scarf, gloves, tuque, warm shoes, thick trousers (I often put 2 on). Don't wear this inside plane yet. If you have good circlulation and good posture you get less cold as well, and staying in movement also helps. For increased blood circulation you can try hawthorn (e.g. tea), active ingredient rutine but look up about it first before using.

In Europe we might have some different strains of diseases like common cold and flu so boosting your immune system is wise. There are many options to achieve this; you mentioned some. I also recommend to not eat crap. If you travel more frequently you should have less problems because your immune system is already able to defend itself. Airports are also a good place to get all kind of diseases, and the airco on might also contribute.

Today I was grocery shopping. Someone was giving away papers on street trying to lure people into selling subscriptions. Now, on a business day a paper from kiosk costs 1,20 EUR while the extra thick saturday paper from kiosk costs 2,20 EUR. He was selling a week subscription for 3 EUR. Although a paper does provide backgrounds, news is faster spread via the Internet. We had some talks about when people used a small device to read news instead of a paper, but he was convinced it'd take 10 years till that'd take off, and he was arguing the paper provides backgrounds. The latter argument about backgrounds is a valid concern, on the Internet such is more scattered. I ahreed such change won't happen tomorow, but I expect it to happen quicker than 10 years. So if you buy a tablet with 24/7 connectivity you don't walk around with a paper, you can get your news from RSS feeds (and go to websites which provide news plus advertisements), you can even print it out later, or let it being read before you in car. The advantage is flexibility, speed, cheaper, portable, less environmental waste. So instead of paying 3 EUR a week (good deal actually but the paper was ****) you'd pay the ~13 EUR to your 3G provider. Hence, you're already getting back some of the money you put in your 3G subscription, and you read it on a good screen (compare to phone).