What I have established is a whole class money system, which I call 'Earn and Learn'. When all students are focused and on task, they can earn money, which gets placed in our class money envelope. At the start of each lesson, we have 10 quick maths questions (which is a variety of mixed multiplications). Students can also earn money by getting full marks for these questions, which contributes to the whole class money total. Obviously I do not use real money, but a print out of money specimens on card stock :)
The class can also be fined for misbehaviour or the noise level getting too loud.
Students are working towards earning enough money to spend on 'rewards'. Every so often I will do maths 'art' or something along those lines, which the class has to 'buy into'.