Anne Lamott wasn't so subtle about what she thought of first drafts in her book, Bird by Bird. In fact, she started off by writing, "Now, practically even better news than that of short assignments is idea of shitty first drafts. All good writers write them. This is how they end up with good second drafts and terrific third drafts."
I agree with her. First drafts are roughest, and well, ugliest drafts. It's a common mistake of beginning writers, as well as seasoned ones, to expect themselves to produce shiny, perfect very polished and publishable first drafts. What these writers don't know is that these ugly and every-editor's-nightmare first drafts can provide them with opportunity to explore every angle, every slant, and every idea for an article or a work.
WRITE DOWN EVERYTHING! Well, at least try to write all of them down. This is stage where you don't let memories of your English professors get in way of writing. When you start with your first (or rough) draft, you have prerogative to hurl those boring English and grammar rules out window. Don't worry, you'll pick them up later.
STEW FOR A WHILE! After you get all your ideas down on paper, leave them. And I mean it! Don't even try touching them until a sufficient amount of time has lapsed. Give it a day or two. Let it stew on your desk. Go to your dentist, get a manicure, or write other stuff, but don't, for your muse's sake, get your hands on your first draft just yet!
GET LETHAL WITH THAT RED MARKER! The next day is time you can brandish your most lethal weapon - your red (or any color you prefer) marker. Take out your terrible (and you will realize that it is terrible!) first draft and start crossing out ideas and sentences you don't need.