This is similar to what I've found, in that writing documentation for a product before I've written any code actually makes the product turn out better. When in design / development mode, I tend to think of all kinds of "cool" features to put in, but since using them requires knowledge of my state of mind at the time I designed them, it gets very difficult to document these features.

So I end up writing the end-user documentation, then build a specification from there (functional requirements), then work on the technical design while prototyping various elements. Stringing together the prototypes often then ends up in a finished product.