Start using math as soon as the pupils arrive in school. Get pupils to stand in groups, while an elected member of the class counts the different groups. You could even make a chart inside the classroom to record the totals. Try incorporating math questions throughout the day in other classes. For example, in gym class they may be playing baseball--you may ask the children to answer various math-related scores or statistics.
Incorporate children learning math into story time. There are many children's books that involve counting. A great book for this is "Blueberries for Sal," by Robert McCloskey. Within the book there are questions that you can ask the class. For example, you might ask "How many coins did [insert name] drop into the tin can?" You can also offer prizes to the pupils who answer correctly.
Suggest to the children that they count out their body parts. For example, "How many feet do you have?" Then ask them "If you stood with your Mom and Dad, how many feet would there be all together?" Alternatively, you can ask their age and direct them to use their fingers or toes to count with.
Use math games online. There are many resources for all ages available online aimed at teaching children simple math. Most games involve simple counting, tessellation and fitting shapes inside one another. Try 'MathProf' for games like these.