Be prepared to feed, change, hold, and entertain him/her for 3 times as long as the flight is supposed to take.

Please, I beg people who travel with infants, get the child a seat and put your car seat in it. The child will be happier and all concerned more comfortable compared with "lap child." But the main point is: it is easy for a child to get hurt in more than the usual turbulence.

Ditto the good stroller. We always used the one that our car seat mounted into and never regretted it. Well, in one way: we would have been better off with a jogging stroller.