The Dreaded Outline
First: I never actually finished that story. My word count goal was 15,000 and was way too small to actually finish a fantasy story! It's currently at 121 pages and over 56,000 words.
Second: My character was all over the place (see "Consistency in Characters" for how I dealt with that).
Third: How was I supposed to have a perfect ending if I didn't know what the rest of the story was going to be about?
The point of NaNoWriMo is to write a story in a month, and I interpreted that for 3 years to mean that I was supposed to sit down and write whatever came into my head. The plot of my story has changed over 10 times over the past 2 years I've been slaving away at it, and is now completely different that what I imagined it would be.
You MUST write an outline. Unless you are a rare genius writer who can keep the entire plot in their head and stay consistent (they do exist, but I am not one of them), an outline is the most essential part of a story. I didn't write an outline, and because of that, I am now scrambling to find a solid plot for a story I started in November of 2012. I have since made an outline for that story, and it's slowly coming along.
My outlines are literally just paragraph after paragraph of horribly constructed sentences that say exactly what is going to happen in each chapter (I leave chapters undefined though, I'll start a new one when necessary). I do say things such as "time passed, Sylvia still sad over Peter" and then let future me decide exactly what will happen, but for the most part every major event is detailed. Most people I know do an orderly bulleted outline with roman numerals and such. You can also do a chapter-by-chapter outline that is particularly helpful if you want to plan length of your book (you'll find that you tend to make chapters the same length depending on your unconscious preference, mine are around 10 pages).
The single tip I can give you is to make an outline. They are a pain, they will make you labor over every part of your story and cause you to question why you're even writing it, but it will pay off. When you're actually writing the story, you will fully appreciate the outline for all that it is.
Outline or no? Let me know your thoughts on outlines in the comments below!