All the mask windows begin to eat up loads of screen space and start to drive your window manager batty. The quick and easy solution to freeing up screen real-estate without losing track of what mask is under what "untitled" window, quickly zoom-out your images with a couple taps on the '-' key, and then shrink-wrap the view with Control-E.

If you are going to work on an especially complex image, save your masks as you create them to descriptive filenames, prefixed by something easy to sort, like 'm-'.