Jerry Pournelle

To doubt everything or to believe everything are two equally convenient solutions; both dispense with the necessity of reflection.

I am sure it has been done with less, but you should be prepared to write and throw away a million words of finished material. By finished, I mean completed, done, ready to submit, and written as well as you know how at the time you wrote it. You may be ashamed of it later, but that's another story.