I happened to run across a great document by David Cornelson of TextFyre called "Writing for the Interactive Fiction Medium." The first 10 pages covers a lot of the basics of how to think about the creation of rooms, objects and interactions. But what's was really eye-opening for me was the next part which was a high-level design document describing TextFyre's game The Shadow in the Cathedral.
Although it covers only the opening scene of the game, Cornelson provided a much clearer illustration of how to think about and plan IF better than just about anything else I've read. The style of the design document is a pseudo code that is even higher level than Inform7; so it is very readable and yet also close enough to the implementation language as to still be useful. Cornelson wonderfully describes the scene, objects, commands, characters and events that kick of the story. The design document illustrates the power and the challenges of programming IF while reinforcing the importance of the story. I found it very inspiring.
You can download the document here: Cornelson - IF Writing Guide