Kind of a neat approach — visualize the book review you want to read before writing your book:
Many years ago, David Allen shared with me that one of the first things he did when planning his first book, the best-selling, Getting Things Done, was to write the Wall Street Journal review of his book, first. He wrote the book review as he would like it to appear in print, even before writing the first chapters of his book.
Derek Scruggs adds:
An acquaintance of mine, a direct marketing guru, once told me that he writes the sales letter before he ever creates the product. Only after he’s explained exactly what you’ll get and why you need it does he set about creating the product. (And sometimes, if the sales letter isn’t compelling enough, he just abandons the product altogether, saving him a lot of time and effort.)
The GTD blog also has the helpful reminder that if you don’t know what "done" looks like before starting a task, you won’t know when you are done.