First off, I've hated outlines for years. I've only recently come to appreciate their virtues. The reason for this is simple. To me, the act of writing is the act of telling myself the story. (It's one reason I can speedwrite so effectively -- I don't know what's going to happen next, and I'm eager to find out.) That anyone else gets to enjoy the resulting story is a function of my overtrained subconscious and my incipient carpal-tunnel syndrome. So writing an outline very much has the feel of taking the air out of the tires of the novel itself.
Second of all, the definition of "outline" is very elastic. To belabor the obvious, we aren't talking about a term paper outline with sections and subsections. We're talking more along the lines of a synopsis, something that tells the story in a summary sense. (See my prior objection.) But that can be anything from a few paragraphs on a page to fifty or more pages of detail, depending on the writer, the idea and the publisher's requirements.
For a while, I had a tendency to write the novel first, then write the outline after the fact. I honestly think this is a legitimate technique, but I've also broadened my repertoire.
Here's the crux of it: my objection to outlining was essentially a short story writer's objection. Though my span of control now seems to extend to about 200,000 words, based on recent experience (and didn't that make the wax run from my ears), the art of formulating and writing a novel is substantially different from the art of short story writing. This was the basis of snurri's recent musing on the topic, variously answered by me and others.
I cheated with Rocket Science [ Clarkesworld | Amazon ]. It's basically a giant short story, in terms of structure, scope, character development, etc. It works pretty darned well, if I may say so, but if you analyze it according to snurri's line of inquiry, it falls over as a novel. And I wrote that book as a short story writer, with a short story writer's approach.
What I'm able to do now with outlines, and this is hard-won wisdom for me, however obvious it might have been to others, is outline first in order to provide the direction and complexity to drive to the depth of detail a novel requires. The outlines for the two most recent books I've drafted, Trial of Flowers [ Clarkesworld | Amazon ] and Madness of Flowers, helped me maintain the byzantine plots and tangled characterizations the books called for. More importantly, the outline for Stemwinder allowed me to sell the book concept to my editor.
In effect, learning to outline was for me part of the process of learning to rewrite -- I tell myself the story once, in brief, then again in detail.
Bonus Question: What does an outline mean to you? How do you approach it?