The best way to shape a child's behavior is to notice them doing what you want and call attention to it. Catch them doing something right. Compliments put a smile on your face and encourage you, right? Build others up, the Bible commends in Ephesians 4:29, "that it may benefit those who listen." Well, children benefit greatly when you applaud them. Stepparents, when you compliment a child you get the added benefit of strengthening your relationship with the child. Double-bonus!