I've always gotten a bunch of sandwiches and snacks from a place I like before an intercontinental. Airplane food in variable at best, and it's nice to have something familiar. Also, once you pass security, get some bottled water. I always, always, carry a toothbrush and toothpaste with me. Even for a 4 hour flight, refreshing yourself makes you feel so much better. I used to like to fly at wierd hours to get on a half full flight, but with all the cutback these days, you'll be lucky if the flight is 80% full.
I will usually not drink much on a long flight either. Planes dehydrate you anyway, and I find that more than a couple of glasses of wine will give me a headache. I wear what I'm most comfortable in on a long flight, which means, for me, jeans, sneakers (usually low cut for long flights, for obvious reasons) warm socks, a tee, and a sweater. Sometimes I keep my most comfortable (and remember that comfort is as much psychological as physical) jacket with me, because when you fall asleep, you can feel very cold, very fast, and I'd rather not use the blankets airline provide.
Oh, and, this should go without saying, do not wear your contacts unless you hate yourself that much.