Establish clear expectations for the children. Some kids have a hard time with discipline because they aren't clear about what kind of behavior is expected from them. Give them guidelines to follow that are easy to understand and implement.
Be consistent. If a child misbehaves, ensure he always must deal with the consequences. Children naturally test boundaries to determine exactly what they can get away with. If a student insists on testing you, help him understand he will get the same unwanted result every time he breaks the rules.
Be fair and equitable. Don't show preferential treatment to certain children, even those who tend to perform better academically. If any child breaks the rules, enforce the same punishment every time, no matter who commits the infraction. Don't regard a child differently based on his attitude or ability to follow the rules, either. Treating each child with the same level of importance and value reinforces a child's inherent value as a person and will help him respond better when correction comes. Children will act out against perceived inequality, especially when it comes to discipline.
Maintain control of your emotions. Avoid arguing or escalating a confrontation with a defiant student. Maintain a firm, authoritative tone of voice, but engaging in a shouting match with your student will not be productive or lead to any sort of preferable outcome.