I'm not getting anything done today--after not sleeping well one night, I had a night of REALLY not sleeping well, courtesy of my decision to find out what the big deal is about Starbuck's Pumpkin Spice Latte. My report: It tastes truly horrible (avoid unless you just love artificial flavors), it hurts your stomach, and the stupid barista won't give you decaf when you ask for it. I got some more sleep last night (after taking a Benadryl to counteract all the caffeine I drank so I could stay awake and look after my niece), but not shockingly I'm still a groggy mess today.
The irony is that I had been thinking of doing a post on what you can do on days you can't write, which was somewhat inspired by this post by Midge Raymond on the Creative Penn. That one's pretty vague, and honestly I don't get the PROMPT thing, but I liked the idea.
Obviously, there are the beta tasks--whatever marketing and production chores you may have on your to-do list. But if you're in the middle of writing, and all of a sudden circumstances force you to stop, there's other stuff to do that can help when you get back to it.
When I start out, things tend to be rough, and sometimes (as happened the day after I wrote that post, in fact), not being able to write for a day can be helpful, especially if you can spend the time thinking about what the problems are in what you've written and how you can fix them. Often when you're writing something and it's off, the emotion ("This isn't any good") registers, but that's so general that it's not useful and can even be discouraging. Actually taking the time to sort the problem through analytically (what, specifically, did I not like about that part) leads you to solutions (well, in that case, I should do X--and, hey, that solves another problem, too).
I also find the thinking-writing-thinking-writing process more useful than trying to plan out everything at the outset (although I definitely do plan), because as I mentioned in the comments, a lot of stuff doesn't become clear until you start writing it out. In that case, there were pacing problems, which are not going to show up in an outline. I also find that in an outline, I'm more likely to have a character do something just because the plot needs for it to be done. Once I start writing in a character's voice, I deepen my acquaintance with that character, and that helps me either to make sure the character's actions are organic to their personality, or to realize that the event needs to be triggered in some other way. The writing drastically improves the thinking, which is yet another reason why you do have to start putting words on a page if you want to get anywhere.