Reward good behavior.
Ah, the power of a bribe. Things not going so well? Give your children an incentive to clean up their act. No complaining for the next hour yields a stop at an ice cream stand. Leaving your sibling alone gets some one-on-one time with Mom or Dad in the pool that night. A whine-free morning means picking where to eat for lunch or which attraction to see first that day [source: Buckworth].
Use this strategy in reverse, too. For example, kicking the back of Dad's seat after being told to stop repeatedly means no trip to the hotel game room that night.