I have trained Nanook that way, with Raustyk & Kaige it just kind of came natural as neither one of them are roamers or the like. I know many others who have trained their dog to stay in the yard as well.

It is pretty simple, it's just like training a dog to be in an invisable fence except with no shock collar or the like. All the inv. fences are are just training tools anyways, they are obviously not real fences. lol