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http://tipnut.com/how-to-train-a-dog-to-potty-in-one-spot/

I've had luck (similar to what the article suggests) to putting them on a leash to train. If they don't like the leash/won't go on a leash I do go outside with them and call them back to the spot until they go. They associate the smell of the area and keep to the same area. You do have to keep the area clean though, if you have not picked up in a while the spot will keep getting larger (we are bad about this in the winter).