Sweet Incentives
Many teachers like to use Sweets or Candy as Incentives for their students, especially with younger classes.
The idea is that if a student does something right then they get a reward. This is fine, but here is an alternative take on that which is especially useful for very small classes or one-to-one lessons.
Suppose, for example, you are trying to enforce the English Only rule with your student and they keep breaking it by speaking their MT during the lesson. Quite simply arrive with a jar of, say, 10 sweets and put it on the table at the beginning of the lesson. Explain that whenever they "break" the rule then you take a sweet out; whatever is left at the end of the lesson they can keep.
So you begin with 10 sweets and then, if the student says something in their MT you take a sweet out (ostentatiously, of course) and put it in your bag. Then if they speak again you take another... until either the lesson ends or there are no more sweets left!
This incentive works well with some kinds of rules and some kinds of classes but a similar approach can be worked up depending on the cirucumstances for most classes. It's just a matter of finding something specific to control and something specific to act as a reward.
