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.
I suggest a simple answer: Treat "drop-in-motion" as a fluency. Let the dog learn that you may require him to drop while moving any time, anywhere. To do this, of course, the down first has to be established as a fluency. This may sound too obvious to bear stating, but you cannot train the drop-in-motion unless you have the drop in place. Many new Open trainers find themselves in a mess precisely because they have not really taught the down except as a stationary (stay) exercise, and at a close distance. Since the down is not fluent, it cannot be added into any more complex exercise.
There are myriad ways to shape drop-in-motion. Much will depend on the resources available to you. One friend of mine took advantage of the physical layout of her house. Her puppy could do laps through the living room, down the hallway, around into the dining room, and back in to the living room. Being a puppy, the dog would really fly, and my friend would stimulate her by stamping her foot. Then suddenly, in the middle of one of these "zooms," she would say "Down!" Click! Treat! Her dog developed a really reliable—and fast—drop on recall.
You can also add the "drop" with the dog walking at your left side. As you are walking, say "down" and pivot directly in front of the dog. Don't stand right over him or he will be likely to sit instead. Give him a step or two of room. Be ready with the target stick to remind him what he is supposed to do if necessary. C/T the drop. The reason to pivot directly in front of the dog is to stop forward motion. You want the drop to be "in place," and immediate.
When the dog is reliably dropping while moving in a small area, expand the area and ultimately get him outside.
You have two primary reinforcers with the drop-in-motions: the treat, and the game of the dog jumping up on cue and running to or after you. This exercise allows for a lot of play, so give the dog plenty of it here. Remember always that the PR is for the down, not for coming to you afterward. When you want to add that element, do not click for the down but for the secondary recall.