For short flights try and eat before you get on the plane or if you're in a hurry grab some food in the terminal and eat on the plane. This means you can have something healthy and use the flight to catch-up on some sleep - most of us don't sleep enough so cat napping is a good use of a flight. Airports have a much better selection of food than they used to, I usually pick something up at Pret or EAT and fresh sushi is often available. A lot of airports now have juice bars so take the opportunity to top up your vitamin C and ward off plane germs with a fresh fruit or vegetable juice.
For long-haul flights aeroplane food is often unavoidable. Ordering the gluten-free or vegetarian options will usually get you a healthier option so worth doing even if your not veggie or gluten intolerant. Vegetarian food is often easier to digest than meat so can also be a good idea given it's often difficult to sit properly in a plane. Always skip the pudding, unless it's fruit, and instead pack some healthy snacks to keep hunger at bay. All the snacksbelow have a long shelf life so pack enough for the return flight:
Dried fruit (check no added sugar)
Nairns oat cakes
Laar or Fruitus bars
Bounce protein balls
Avoid alcohol, tea and coffee as these will dehydrate you. Instead ask for water when the drinks trolley comes round and try and drink plenty of water before you get on the plane and as soon as you get off.
If your flight is long enough for more than just sleeping (the best use of a flight if you can get comfortable) use it to have some relaxing me time: catch-up on some non-work reading, listen to some of your favourite relaxing music or make the most of the inflight entertainment and watch your favourite sitcoms or enjoy a movie. Go for comedy over drama/horror - laughing is a great stress-buster and anti-ager whilst movies which evoke negative emotions such as fear or sadness can encourage the production of stress hormones in the body.
Always pack your flight socks and take a walk around the cabin when you wake up, also pack some anti-bacterial wipes to clean your hands before eating. Ear plugs or good sound-blocking ear-phones are a good idea to keep your trip much more relaxing - especially if you get sat near a toddler!