Long distance flying is a circus of misery!
I agree with the other posts, despite the fact that I have an open bar and nowhere to drive, I avoid alcohol. Dress REALLY comfortable and don't be afraid to ask for anything you need from the staff. I'm 6'6" in my bare feet, so imagine how miserable an airplane is for me!
The air in the plane seems like it has about 2% humidity, so drink more water than you think you need. It will keep you hydrated and give you something to do (and a good reason to get up). If you wear contacts, take them out for the flight.
Don't forget to check your devices for voltage requirements and to bring your outlet adapters. Though it's rare, some devices are not dual rated. The UK and Ireland have a specific outlet adapter, continental Europe is different (the Germans call it the "pig snout"). Both are 50Hz (US is 60). I make mulltiple outlet adapters by buying replacement plugs while I'm there and using an American multiple outlet strip (without surge protection), but I'm usually with a group.
I would also get local currency ahead of time, it's a bummer to be stuck in the airport with no way to get a snack or something to drink. Credit cards are not as widely used in Europe as they are here. Phone your credit card company and let them know you are traveling international so they don't shut off your card for strange transactions. I always bring pound notes and Euro, regardless of where in Europe I am going. It has come in handy a couple of times.
NEVER leave your passport unattended. I can get to the airport in Europe with my charm and good looks, but it doesn't work on Customs and Border Patrol when I land in the US.