The Envelope System--for budgeting your paychecks and staying on budget. This was actually how I saved up $630 for a computer and $350 for a new aquarium at the same time... then I stopped doing it. Maybe I should start again.
Stop negative behavior in your classroom before it starts by reinforcing your expectations and recognizing positive student behaviors. With two simple tools, I successfully managed student behavior at the secondary level. A ticket system and "shout outs" are little to no cost, require little work for you as the teacher, and will encourage on task, positive behavior from your middle school and high school students.