My poor DD has flown more than 10 times with us, and over half were international (she is 20months, too).
Everyone's advice is great, I would like to add 2 more.
1. Go to a charity shop and get a bunch of new (well, new to YOUR child) little toys, and get some strings to tie to each one, that way when your sweet child just happens to 'drop' the toy 10 times, you don't have to try to fish it out in cramped quarters. (also, it becomes it's own game, which is easy and fun)
2. Take your child to the back of the plane, where there is a little area to stand. On almost every international flight, I met other parents with kids, and you know how kids love to look at each other/'play'
Have fun!