First of all, I always weigh my bag before leaving for the airport. I make sure that my bag’s weight is at least a kilogram less than what's allowed, allowing some margin for any weight mismatch between my machine and the airport’s. That way, I don't have to unpack my suitcase on the dirty airport floor with hundreds of strangers looking at me while I frantically rearrange the bag and throw away valuable clothes!
Then, I always try to visit the airport lounge for a quick meal. If the meal isn't included in my flight, that lounge meal keeps me to remain full, and I don't feel hungry, and otherwise, if the meal is included, I still eat something light so that if there's a delay, I won't feel hungry. That way, I don't have to rely on the expensive airport restaurants if I get hungry. Of course, this is only applicable if you have a credit card that offers complimentary lounge access.
If it's a long flight, I usually pre-book an aisle seat because that gives me some leg space to move my legs a little. Also, I try to board the aircraft sooner so that I get space in the overhead bin to keep my handbag. Otherwise, keeping them below the seat eats away the limited leg space I have, and the journey gets very uncomfortable with stiff legs. It's usually a window seat for shorter flights and an aisle seat for longer ones.
Some people use a neck pillow to get some sleep. I have purchased one, but that never helped me, so I have stopped carrying it. Use it if it helps you, but it doesn't work for me. On the other hand, I keep a lot of movies downloaded on the OTT platforms I use so that I have something to watch during the flight and avoid boredom.
Also, the aircraft cabin can get chilly sometimes. Not every airline will provide you with a blanket! So it's better to wear full-sleeved clothes so that you don't feel cold. And it's important to keep yourself hydrated. So drink water as regularly as you can. I sometimes carry an empty 1-litre bottle of water, which I fill from the water fountain after the security check. Asking for a cup of water every half an hour inside the aircraft may feel a little awkward.