Repeat your directions. If necessary, phrase them in different way. Defiant students often need to be asked multiple times before completing a task.
Develop a reward system. Providing students with incentives (stickers and snacks are useful options) for appropriate behavior will help prevent them from acting out. Students are less likely to act defiantly if they know they stand to gain from following directions.
Develop consequences for poor behavior. Make sure the discipline given suits the poor behavior and that punishment increases with each episode. "Timeouts" and privilege suspension are common disciplinary options.
Contact the students' parents. If students continue to act out despite repeated consequences, parents should be informed. Explain the student's behavior and discuss further action that might encourage better behavior.