I'd agree with the "you can't make them do anything" sentiment, as well as the advice about using a bottle, and making sure they are hungry.

One other thing I've found useful on our two intercontinental RTs (at 3,5 months, 4,5 months, and 8 months), is to use a baby carrier. First the baby has to be familiar with it and comfortable in it, so have to start well before the flight (I've used them since my LO was about 1,5 months old), and have found having baby in either a pouch or a soft-structured carrier to be fantastic for both me (hands free) and calming the baby.