Here are some things I always have whenever I travel:
1. Water bottle - long flights tend to dry you up and you may end up with a sore throat (and other sickness) if you don't hydrate. I always request the stewardess to fill up my water bottle at the beginning of the flight and they always indulge refills as well.
2. Extra shirt and face towel - a change of clothes mid-flight help keep the blues (and the baho) away.
3. Toothbrush and toothpaste - not all airlines provide this so it's best to have it in your hand-carry. (Also try to bring some mouthwash if you can).
4. Lozenges - sometimes even when you drink lots of water your throat would dry up. Lozenges help.
5. Entertainment bag - you can only watch as much movies and TV shows throughout the flight. I always bring a tiny quilting kit with me so I have something to do with my hands. I also always have a book with me.
6. Pen - for filling out immigration forms.
7. Compression socks - to keep DVT away!
And one thing I try to do is assess what I really use during long flights so I can trim down what I put in my bag.