I'm still making the slow and sometimes rather painful transition from a full-on pantser to an outliner. At heart, I like to let stories come together on their own over time, unfolding as they will, but now that I have deadlines to meet, having an outline—even one I know I might deviate from—saves me a lot of stress.
The changes aren't just about the outline, though. I used to be on a very need-to-know basis not only with what was going to happen in the book, but with the back stories of my characters and my worlds. This, too, has changed. I still discover little (and not-so-little!) things about my characters and worlds as I write my drafts, but knowing these sort of things up front, like having an outline, saves me pain in revisions when background information doesn't match up.
However, the panster inside me refuses to give up, so I've adopted a sort of half-and-half technique, especially for completely new stories.
Much of the time, I start writing an entire book with no more forethought than a single scene. A single conversation. Hmm.... Okay, so there's this boy hidden in a tree, trying to keep quiet as this mass of white-hooded figures glide through the woods below him, moving to some unheard music. Suddenly, one of the figures stops moving. It's a girl, snapped out of some kind of trance. She begins freaking out, trying to push out of the mass of humanity around her. The boy watches, mystified and helpless...and that's it. I know nothing else, not even the characters' names or why they're in the woods or what the heck is happening with the girl or the white-hooded people.
It's a fun way to write, and I love learning about the story as I write it, developing the plot and the characters as I go along. But it's a tangled process, and more often than not, I get caught up in one mess after another, writing myself into corners, and realize that I don't really know the motivations behind my characters' actions or the laws of my world.
I still write that way a lot, though, because I enjoy it so much. The difference is, once I've run off that initial OMG-new-story! high, I take a look at what I've written (which is usually not more than 10k words or so) and then start a new document. This is the world-building doc. There, I write all the "causes" to my "effects."
I don't think there's a right or wrong way to go about making a doc like this. I like to write about worlds unlike our own, so I start out by outlining the important history of this world. Basically, what has happened in the past lead up to events in your books? Then I write about the current world (what laws does it abide by? what is the social structure? where do the resources come from?). This doc also involves my characters (who are they? how old? what do they look like? who is their family? what are their motivations?).
I don't know about you, but I actually adore this kind of stuff, so I can go on for 10k or more just in the world-building doc alone. It's very useful--not only for checking up facts to make sure they stay straight in the book (what color eyes did that minor character have again?), but also to make things more concrete.
Also, typing out all the world-building and character information helps me see where potential plot holes lie and where I needed to strengthen motivation or some such.
In this way, I get to satisfy my panster-heart (my laptop autocorrect keeps wanting to turn "panster" to "punster." What on earth is a "punster"? one who tells puns?...that's exactly what it is, isn't it?) AND keep my facts straight enough to meet my deadlines :)
Do you keep world-building docs?