We did an 8 hour flight with 16 month old last year. Just repeat to yourself that you are there for the sole entertainment of your child, no more relaxing eating and watching a movie! That was the hardest thing for us to cope with! Of course I'm saying "we", hubs did get to do all that which we had a "discussion" about once we were off the plane.
We gave/give him lots of freedom before getting on the plane. We didn't do this but I've seen recommended that if you're travelling with another adult they take the luggage on the plane early and the person with the child waits as long as possible so the child isn't cooped up as long.
We brought lots of new little things to play with like has already been suggested. Also items of comfort, obviously! Son had his dummy for take off and landing for his ears, if there's no dummy sucking on a bottle is supposed to help.
Dry snacks that take a while to eat are also good entertainment. Son has never wanted anything but dry food on flights.
Stay calm yourself so she doesn't pick up on any stress and start acting up. Even if she cries a lot remember it's public transport and she's as much right to be there as on a bus or a train and if people don't like it it's tough, this is life. See I have to remind myself of this when I get my kid off to sleep and someone with a miserable kid walks them up and down the plane crying their eyes out which then wakes up my kid. Stay calm... they have a right to be here too, they are just doing their best!
Can you check to see how full the flight is so you'll know if you're going to have space or if she'll be on your laps the whole time.
And remember, time shall pass... the plane flight will eventually end!