When do you hill potatoes?
I actually don't at all. I'm too lazy. I just plant really deep to begin with. And we get a pretty good yield every year. There usually are a few near the surface that turn green, so if I were to hill at all I would add a thin layer right around when the plants flowered - that's when they start forming tubers, or so my gardening books tell me. I also hear that you can just use straw or hay (grass clippings might be too nitrogenous, though) on top of the ground.