One of the really nice things about InDesign is its liberal policy of undo/redo. It invites experimentation.

Ever experiment yourself into wishing you’d started on a master page instead of a document page? Say, maybe you put together the perfect page, using objects on multiple layers, but want to apply it to the whole document? You could select all (CMD+A [CTRL+A]), turn on Paste Remembers Layers (Layer palette menu), create a new master page (New Master from the Pages palette menu), then Paste In Place (CMD+OPT+SHIFT+V [CTRL+ALT+SHIFT+V]).

Or, you can just select Save As Master from the Pages palette flyout menu. Instant master page from the current page.