My personal favorite is to tell a new FA that the aircraft is very sensitive to lateral changes in Weight and Balance and that it is of the utmost importance to remain as close as possible to the center of the walkway. Have the NFP watch her as she moves away from the center; PF rolls the plane in that direction. Works every time.

Another fun one is to have a new FA ask for "permission to come aboard" every time she enters the flight deck. This is most effective if carried out for an entire four day trip and then allowed to continue on to whoever she flies with next...