I don't usually notice jet lag when I cross 2 or 3 time zones - but once you cross 6-12 hours of difference - say NewYork to Kenya, jet lag can really be an issue. The first time my wife and I flew to Africa for safari, we stopped in London( 5 hours east of NY ) for 3 or 4 days to explore, and get resynced, and then flew on to Nairobi - worked like a charm. We felt great in Nairobi which is 3 hours east of London. I had some friends who flew from LA to Nairobi as directly as they could, and it took them almost 4 days of the safari to get themselves upright and enjoying the vistas.
Hydration really is an important factor for dealing with Jet lag.
I always take a couple baby aspirin a day before flying to reduce the risks of blood clots with deep vein thromobosis as well as trying to stay hydrated. I bring along some baby aspirin and some ibuprofen in my carryon, in case I might need them.
Comfortable but supportive slip on shoes are very desireable. The last time I came home from Europe I walked over 4 miles inside airports carrying over 50 pounds of bags. Carrying a bag of cement 4 miles can be tiring and one needs good shoes. I am glad I was not wearing high heels like some of my fellow passengers,
Standing up and moving a bit, after the seat belt sign is turned of if it is, also helps keep your blood flowing normally. I have a sneeking suspicion some airline personnel don't really want their passengers up wandering around, and tend to not provide enough water unless they are prodded - ask nicely and politely. Ask more than once if needed.
I like chewable vit C in 500mg tabs. I like the ones from Nature-Made.
I always keep a soft dark well worn cotton bandana in my briefcase to help cover my eyes from external light so I can doze off if I want to. I keep a microfibre cloth in my briefcase also, to clean my glasses, or to remove fingeroprints from the window if I want to photograph out of it.
If you wear glases, bring a second pair in your carry on. You might find you need them before you get back home, and they will be hard to get replaced quickly in some areas of the globe.
Take a picture of your passport with your phone, upload an image of it to a secure storage area in the cloud, so if your passport goes missing, you can help the embassy personnel get you sorted out more quickly.
Any medications you need to take daily, should be with you inside the airplane in your carry on bag, not lost in luggage in Idaho, or somewhere. It is probably better if the drugs are in their original Rx bottles, but I have never had an issue with a small bottle of pills containing all my medications for a 2 week trip - but I am an old man, and I do not carry recreational drug type items, nor do I look like it would fit that profile. Use your own judgement.
Be smart, be polite and cheerful to customs officers. Make their day a bit more pleasant, and they might reciprocate.
Travel is one of my greatest joys, but not all parts of travel are always fun, and airports aren't as much fun as they used to be many years ago. I can remember getting to look out the cockpit windshield in flight on a commercial flight from Kansas City to Chicago when I was an adolescent. Long time ago in many ways...