Let your class know what your expectations are for its behavior in regard to sitting. Set rules at the beginning of the school year and consistently reinforce them.
Provide each child a carpet square to sit on during reading or circle time. Having their own sitting space will remind your students to remain seated.
Allow your students to take breaks. For instance, if you are working with younger preschoolers, take a break in the middle of circle time to do some stretches. This will increase the likelihood that your students will succeed in meeting your sitting expectations, which in turn raises their confidence.
Provide small, unobtrusive textured toys to children who have a particularly hard time sitting still. While it seems counterproductive, the sensory input these toys provides can help the child focus.
Notice and reward good sitting behavior. Acknowledging your students' successes will increase the likelihood that they continue to display the desired behavior.