LOTS of good suggestions here! I'm leaving for my fifth trip to Europe on Monday. It's about a 9 hour flight, not counting all the waiting around in the airport before. What I've learned about long flights:
1. Loose fitting clothing is THE most important thing, in my opinion. I don't know what 1st class is like, but in coach you are CRAMPED. And you will be miserable without soft fabrics, elastic, etc. In the winter I wear a stylish dark colored jogging type outfit, pants and jacket, with a comfortable teeshirt underneath. Nothing tacky, but nothing fancy. You shouldn't look like a slob while flying - my grandfather was an airline pilot, and we've been instilled with the feeling that you 'dress up' to fly, and on flights between Belgium and Italy, I do dress very nicely. But transatlantic? Clean and comfy and neat is best. Your body swells with water retention because of the altitude and dryness and it is uncomfortable. Since it's so hot now, I'm planning on a nice, elastic waisted, floral linen skirt (just below the knees), a nice teeshirt, a cardigan, a denim jacket, and sandals for monday. Putting knee high wool blend socks in my purse because sometimes it gets really cold.
2. Water. Some people will put this at number 1, but I can handle the thirst more than things digging into my skin and torturing me when I'm trying to curl up. I drink a one liter bottle I bring with me, plus take a glass from the attendants whenever I can get one.
3. Something to eat. Most US to Europe flights, on the east coast at least, leave at night, to arrive in the morning, and the first 2 times I flew I made the mistake of not bringing food. I cannot and will not eat the suspect, sodium and fat laden airline food, and I'm not a big nighttime eater. I STARVE in the air, I don't know why. I bring different healthy snacky things now, something for the evening, and something during breakfast service - even though your body thinks it's 2AM, they're serving eggs and it's light out, and you get grumbly. My food this time will be sesame tamari rice crackers, cheddar cheese, and green grapes.
4. Something to read. I get waves of motion sickness and can't read much (can't read in a car at all) but it's good to have. An easy guilty pleasure book or magazines are best, because of all the distractions around you. And the movies are crap, but I usually wind up watching when I'm too tired to read, yet can't sleep. I can never sleep on planes.
5. An extra pillow, if you can manage it. I always beg an extra from the attendants, and need one behind my neck, and one behind my low back. Those coach seats KILL.
6. Make sure you get up frequently - at LEAST once an hour, preferably twice, to stretch your legs. Walk the aisles, hang out (by yourself, if you talk to someone you get told to sit back down, after 9-11) by the bathrooms and do leg stretches that will cut down on the risk of developing a blood clot.
7. Don't drink. However tempting those mini bottle of wine and liquor are, they will dehydrate, and they affect you MUCH more than on land because of the altitude. Believe me. Unless you're a lush, you'll regret even one

I had one mini bottle of red one time, thinking it would help me sleep. It did, but I woke up about 3 hours later RUNNING for the bathroom. I don't think I've ever been that nauseous in my life. I think it was the combo of alcohol and motion.
8. Wear a mask to breath through if you have allergies, get sick easily, or just want to. Lots of people do it. It's no big deal, and you shouldn't be embarrassed. The recycled air is HORRIBLE. I generally wear one for at least half the flight - when they serve peanuts, I'm deathly allergic, and when anyone is coughing or sneezing. Rather have a couple raised eyebrows than come down with something or have to use my epi-pen.