Even though I'm not a "professional" writer yet - though I did get to deposit the first check I've received for my writing since a competition in high school this week - I treat my writing as though it is a job. As with any job, there are deadlines that I have to meet. Only, in this case, most of the deadlines are made by me.
Treating your writing, from the outline you've just started to the re-write of draft number five, as a project that you're getting paid for (even if you're not) can actually be a great motivator. I've found that since I adopted the "every script is a job" mentality, I've become more focused in my writing, sharper and more professional. The writing is of a higher caliber, from a more experienced writer. All too often, I fell, when there isn't a concrete drive to deliver a high quality product, quality can inadvertently fall by the wayside. That's not to say that I want to write an inferior script, but by treating the work as something that someone is waiting for and expecting a high quality result from, it helps me focus on delivering.
Deadlines are, so far, mostly up to me still. The producer and manager I'm working with on the post-Apocalyptic spec are pretty loose on when they want the rewrite done by; we're hoping to go out in September. That said, if I claim to be able to get a new intro to them by the end of the weekend (which I did last week), then I'll be damned if I don't do that. Even though the deadline didn't come from them, it's been planted. And at this stage in my "career," the last thing I want to do is come off as someone who makes claims and never follows through. As a young writer with no production history, it's crucial that I deliver on time.
If you don't work with self-imposed or other deadlines, consider giving it a try. The key is to acknowledge that no deadline can be broken. Hopefully, not only will the crunch keep you writing, but it will help you deliver high quality pages, since you know you might not have time to go back and rework them as much. It's an exercise that might help. And, down the road, when you are writing for that major studio, it'll be practice that paid off.