I am no expert but repetition. I have a chessie that I took on hikes this spring. When I found a shed I would throw it for her and play fetch. She never found any while we were out ( well never brought any back to me). But one day while working in the yard she came bounding out of the swale next to the hay feild with a small muley fork in her mouth and ran up to me waging her tail. Hoping this spring she remembers and starts doing the hard work!
