Rapid Prototyping Games
Martin Pittenauer pointed out that Gamasutra has an interesting short report on prototyping a game in less than 7 days. It's mainly about casual website gaming, but they do concern themselves with replayability, and the general ideas ring true for bigger games as well.
Moreover, the general procedure is suitable for application programs, too: Build a 'toy' prototype of the core functionality, fiddle with it until it behaves the way you want, then redo the whole thing for the final app. And artificially restrict yourself (or find an external restriction like a fixed shipping date or whatever) to make sure you eventually ship.