Switching between windows on a computer is a frequent activity, but current switching mechanisms make it difficult to find items. We carried out a longitudinal study that recorded actual window switching behaviour. We found that window revisitation is very common, and that people spend most time working with a small set of windows and applications. We identify two design principles from these observations. First, spatial constancy in the layout of items in a switching interface can aid memorability and support revisitation. Second, gradually adjusting the size of application and window zones in a switcher can improve visibility and targeting for frequently-used items. We carried out two studies to confirm the value of these design ideas. The first showed that spatially stable layouts are significantly faster than the commonly-used recency layout. The second showed that gradual adjustments to accommodate new applications and windows do not reduce performance.