Promoting positive behavior is a two-fold process. You will need to reward the children who behave well, and change the behavior of those who do not. Often in a classroom, teachers give more attention to students who misbehave. By minimizing this, and by rewarding children who behave well, you can use the 'carrot' rather than the 'stick' approach. Treating the children with respect is key to helping them respond well to the changes so your classroom will be more harmonious.
Decide the classroom rules with the students at the start of the year. By involving the children in the rule-making process you can help them set their own boundaries, and categorize what they feel is bad behavior. Talk through the rules as you make them, so that they all understand why the exist. Write the rules on a big piece of card or paper, and place them where the students can read them.
Set a good example to students. As a teacher, you are more than just an academic role model, you are an example. By being positive and cheery yourself, you will encourage the children to do the same. When a child breaks the rules, explain to them in simple language what they have done wrong. Do not become moody or angry, since this will also impact behavior.
Reward good behavior. Each time a child behaves, write his name on a scrap of paper and place it in a jar. At the end of the week, pull a name from the hat and give that child a reward. The more a child behaves, the better his chances of getting the reward.