We have a couple web apps using MongoDB and in both cases we tried to sit and think about how we were modeling our data beforehand. Thanks to MongoDB's awesomeness, it's obviously pretty malleable, so we didn't spend too much time of that but figured "OK, this looks good" and dove in.
In both cases, we had to sort of "back out" our original design, tweak the documents (usually creating new collections), etc.
How could we have avoided this? It didn't bite us too bad but it could have I guess. Obviously, I think the book is meant for people like me, but I wanted to get in a question and see if there's any good suggestions on how to plan out your "schema" and test it, but without actually doing it...Or maybe the best suggestion is to go ahead and use a few tracer bullets/spikes to actually see what is like to work with the data in that fashion.