Graphical User Interfaces present commands visually at particular locations in the interfaces, arranged in menus, toolbars, and ribbons. One hallmark of expertise with a GUI is that experts know the locations of commonly-used commands, so that they can find them quickly, without searching. Despite years of research on GUIs, there is still little known about how this spatial location memory develops, or how designers can make interfaces more easily memorable. One way people remember locations in the real-world is landmarks - so we carried out a study to investigate how users remember commands and navigate in four common GUIs. Our study revealed that people strongly rely on landmarks readily available in interfaces (e.g., layout, corners, and edges) to orient themselves and remember commands. We provide new evidence that landmarks can aid spatial memory and expertise development with an interface, and guidelines for designers to improve the memorability of future GUIs.