Debbie Drown, Educator, Principal, says:

It is not unusual for timeouts to become less effective as children get older. It may be time to have a talk with her about what your expectations are for her both at home and in public. Outline the behaviors that are acceptable and those that are not. Set up guidelines for rewarding positive behaviors and the consequences for unacceptable behaviors and then follow through. If you set up a consequence for an unacceptable behavior be sure to follow through or she won't know you're serious. Rewards do not need to be monetary or tangible in the short term. Verbal praise with perhaps a tangible reward over a long period of time for consistent acceptable behavior is usually your best bet.