This would make a great procedure (Harry Wong). Count to five or hold up one finger at a time when the class needs to refocus. Discuss what occurs for each number at the beginning of the school year and eventually the reaction will be automatic.
New Years Idea: Start the year with an empty jar and fill it with notes about good things that happen. Then, on New Years Eve, empty it and see what awesome stuff happened that year. Good way to keep things in perspective.