With his last rule, Masello advised writers not to let fear paralyze us, because if we do, we won't write, and the fear will lead to regret. (And regret leads to anger, which leads to hate, and then to the dark side...but I digress.)
With this one, Masello warns us about the critical voices in our head that can also lead to paralysis. If you're a writer, you probably know at least one of the voices he's talking about, the voice of the perfectionist that tell you that what you're working on is no good. If you can't get it right the first time, the voice says, why bother doing it at all?
Needless to say, Masello advises us to ignore that voice as well as we can. He's not saying to ignore critical thoughts entirely, but we shouldn't let them overwhelm us. We're going to need to use that voice when we revise our work, but if we listen to it too soon, we might not get the work done in the first place.
I learned a lot about how to quell this voice when I was at Clarion. In our first week, writer-instructor John Kessel handed out photocopies of one of his first drafts, and for many of us, it was a liberating experience. We all knew Kessel to be a well-established, award-winning author, and to see a draft of his filled with comments like "FIX THIS" and asterisks marking incomplete paragraphs -- well, for me at least, it was a revelation. To this day, my working drafts are filled with asterisks and asides. I tend to write non-linearly, meaning that during a writing session I'll bounce back and forth between different scenes, crafting a sentence here and a paragraph there. Kessel's first draft showed me the way. (I previously noted this technique of mine in Robert's Rules of Writing #10: Get Rolling; now you know where I got it from.)
Another one of our Clarion writer-instructors, Ellen Kushner (who blogs on LJ as ellen_kushner) referred to the first draft as a "cookie dough draft," an amorphous blob out of which one could fashion the final story. Again, for many of us, this was an extremely useful idea. One of my fellow students really took to the idea, and she found it helped her make a breakthrough in her writing. By imagining her first draft as the equivalent of cookie dough, she was able to stop being such a perfectionist when she first sat down to write. It would be enough to listen to the critical voices when revision time rolled around.
By the way, I have heard advice on the other side. Literary agent Scott Meredith used to warn writers not to think of a first draft as "just" a first draft, which could be reworked later on. Doing so, he said, leads writers to be lazy and sloppy with their first drafts, and in the end their stories and novels would require twice as much work to revise. Barry N. Malzberg took this advice to heart, and in an introduction to one edition of Meredith's book Writing to Sell, Malzberg noted that almost all his published work is actually first drafts.
But I still feel that for the rest of us, keeping in mind that a first draft does not have to be perfect is an excellent anodyne.