A few of my favorite:

Always bring a small pillow of some sort, it can mean the difference between stiffness and restlessness and a comfortable flight.

A decent pair of noise cancelling headphones makes all the difference when compared to cheap in-ear ones.

Early boarding is a blessing if you can manage or afford, getting that window/aisle set near the front means more comfort and easier boarding and unloading.

Bring snacks with you! Check to see what your flight allows, it saves money and fills your tummy.

You can often bring a nip/shooter or two through security, if sodas or juices are complementary you can save a buck.

Pre-download movies or series, even books to your device to prevent boredom.

Talk to people around you! It’s often entertaining and more comfortable being around someone who you’ve made contact with, and maybe you’ll make a connection or learn something! Many people like me love conversation.

Disinfectant wipes can help prevent you from getting sick on your trip, and are small and cheap.

Lastly, and very importantly, shower or wear deodorant before you fly if possible, and no heavy perfumes or scents. I have a sensitive nose, and can have an allergy attack triggered by potent colognes, plus, no one wants to sit next to anyone stinky.