At the beginning of the day after telling your class what the day will look like offer them the chance for a few minutes (it could be anywhere from 5 minutes to an entire class) to play a game if we have a good day.
If you decide to offer them this incentive make sure you make good on it if they are good!.. and if they are not good do not be afraid to take it away.
It doesn`t even need to be "Game Time" it could be free time to do whatever they want.
I've made this work anywhere from kindergarten to grade 8 (it might even work in high school - I just don't go there often enough to try it out).
A tip for middle school and high school students though would be to have something in mind for them to do... things can get chaotic fast in elementary school let alone high school. Also, in middle school students usually move from class to class. So, if you are seeing one particular class a lot offer it to them for the last class you see them in. If it is a class you will be seeing once that day offer it to them for the last 5 minutes of class.