In the old days, a stern "Don't make me have to come back there and settle it!" was more than enough to silence unruly backseat behavior. But in more modern times, our society looks much less favorably on in-car threats like these -- even though the intent was simply to keep the peace.
One alternative is to offer an incentive for good behavior: the proverbial carrot instead of the stick. Usually you can pick up some type of inexpensive toy from a highway rest stop, and with a little luck it'll be enough to keep the recipient busy in play until you reach your final destination -- or at least the next stop [source: Ogintz].