This may sound overly simplistic, but with my own 4-year old I have found that, starting a week or so before the trip, frequent reminders to her that many people have to share the space on the plane and as such it is not the place for yelling, screaming, kicking (insert your offensive behavior of choice) goes a long way in setting her up for success. In our case this seems to have the effect of diffusing the parent-child dynamic and instead making it more about how her behavior may upset the other pax - it becomes less of a power play between parent and child.

This is advice in addition to the typical - lots of food, toys, books, etc.