To keep students on task use the secret student strategy. Each morning write a name down under the secret student poster on your whiteboard. I laminated mine and put magnetic strips on the back. Tell students that if the secret student was behaved well all day they will receive a prize. If not, don't show the name and try again the next day. Remind students throughout the day "I hope my secret student is on task."
This is great tool for when the end of the year starts getting closer and students start to lose focus.