Well, the idea is to find places where you can get a maximum number of swings without walking. The resources are laid out in an 8x8 grid, so in most every mine you'll find 1 or 2 spots where you can hit 4 spots without moving. Even if 1 of them isn't a really big vein... that is still a good amount of ore at one spot. It's less common to find places like that along the mountainsides.