But this common advice isn't as true as people like to think. If you want to avoid doing possibly dozens of rounds of revisions, you can't just spew out a first draft like it doesn't matter. It can't just be senseless ramblings vaguely connected to your story that you might be able to morph into a cohesive plot later. A first draft is not a bunch of confetti ideas to be strung together during revision--it's a foundation on which you can build a polished story.
Think of it this way: say you're looking at fixer-upper houses. The first hardly has any paint left. The back wall is all but fallen down, and you suspect that the foundation itself isn't in much better condition. The roof is ridden with holes, and the neighbors have reported ghostly noises coming from the basement. The second isn't pretty to look at, either, but at least you aren't afraid it will collapse onto your head. It's shabby, but you know the foundation is solid. It's a bit more expensive to start out with than the first, but it will cost you less in the long-run to fix it up.
You'd pick the second house, right? Of course--it's the more efficient choice.
First drafts work the same way.
You want to be efficient in your writing, saving time without sacrificing quality. If you barrel through your first draft like nothing matters but getting to "the end" by any means possible, it's like the first house. Sure, it's a story by definition. It has a structure. It's complete, if complete is having a beginning, middle and end, in the same way you could technically call the house complete if it has four walls and a roof. It's "finished", but...it's terrible.
You could salvage it, sure. But it would take a lot of time and effort you don't necessarily want to expend. You haven't given yourself firm ground on which to revise your story. You're basically rewriting the entire thing. In that case, what's the point?
If your first draft is like the second house, though, you're in much better shape. It may be shabby, but it's workable. You won't have to destroy it in order to start fresh. You have a starting point, at least. Being a little more careful with your first draft will now pay off in the revision stages.
By all means, give yourself freedom on your first draft--that's what it's for. But keep in mind that any plot, structure, and character problems have the tendency to snowball. An issue with your writing might start small, but if your first draft runs amok, completely unchecked, these problems can quickly become huge issues that make the draft unworkable. Use some discretion in your first draft in order to avoid this. Outlining or otherwise planning ahead of time makes it even easier.
Be careful with your first drafts. When characters get too out of hand, rein them in. Know where you're going. If you know there's a problem, don't let it get bigger. Don't write something you dislike in order to reach a word count. Be free with your writing, and allow yourself to try new directions, but give yourself reasonable parameters. Nobody wants to do dozens of rounds of revisions.
Even so, your first draft will probably be awful. That's okay. Nobody has to write a perfect first draft, and that is, indeed, what revision is for. Still, it will be salvageable. That's the key--not to write a perfect first draft, but to write a first draft that you'll be able to polish.
As with any writing advice, it's important to remember that everyone is different. No two people have the same process. Maybe you're better with first drafts than someone else, or maybe you're worse. The goal should be to find a process that works for you. I'm not saying this to discount my own advice, because I still think it applies--but please always keep this in mind.
How do you keep your first drafts "salvageable"?