From Chapter 3 of Clicker Training for Obedience. While this is primarily useful for the obedience exhibitor, it is an invaluable safety net for the pet owner as well.
One of the biggest bugaboos in competition obedience training is the drop on recall....How do you get the dog to be alert to drop without losing speed on the recall? How do you keep speed on the recall without losing the drop? It is the stuff of which murky legends, rumor, and innuendo are made. One top handler is reputed to hit his dog with his hat. Some handlers use a throw chain. Some now advocate using an electronic collar. The grain of reality in all this is that the drop on recall is probably the most common cause of failure in the Open A ring.