I've done a lot of long flights. My least favorite was over 15 hours. A few recommendations for security: Follow TSA guidelines before you leave. I recommend planning slip off shoes (with socks because touching airport floors are gross). Also have your laptop and clear plastic bag in an easy to grab place so you can wiz through. Be sure to bring all your valuables in your personal carry on. If you are flying United (or another major airliner) download their airline app. You can download your tickets to your smartphone which makes life easy for keeping track of your ticket and any flight changes that may happen (gate changes, flight delays, early boarding, etc.) A lot of airliners now will not have TVs in the seats but instead require you to download their app, log into their wifi, and surf that way. Be sure to bring headphones because you never want to be the jerk who listens to their videos with the sound on for everyone else to hear. Speaking of being a respectful flyer: if you have to pee a lot, request an aisle seat to reduce the number of times your seat mates have to get up. I like this spot personally because it feels less claustrophobic. However, if you prefer more personal space the window is a great choice. Also, when you get up during your flight please don't grab the seat in front of you. Trust me, there is nothing worst than the person who does this!
Other suggestions, look up on youtube how to pack smart. There are ways to save a lot of room in your bags! Plan to drink tons of water before, during, and after your flight. My legs swell a lot so I drink tons of fluids and do calf raises while flying to reduce swelling. Another expert international move is to request a kosher meal. (you must do this at least 24 hours before your flight). You will get to eat before anyone, and won't get stuck with some weird fish and rice option. Trust me, the kosher meals are on point. I also recommend bringing an eye mask for sleep, ear plugs to cut out the sound of screaming children, a toothbrush and tooth paste, and an extra sweater (flights can be either extremely hot or extremely cold).
Make sure to bring a pen with you and information about where you will be staying as you will need it when you fill out your customs form. The flight attendants will not lend you a writing utensil, and the vast majority of people forget about it... so you will get further ahead in the line. Also don't bring fresh fruit, taxidermy, or anything that can get you flagged during customs. Read everything carefully. If you have a layover, try to pick one with at least two hours in between flights so if there is a delay you're not worried about missing your connection. Also, make sure to arrive at least 3 hours early for an international flight. I have seen too many people miss a flight because they ignored the 3 hour rule.
Wow, that's a lot of information. Hope it helps!