While I wasn't a fan of this particular link, for this last story I have used a set of questions that I snagged off the inter-webbie a while ago. The first portion is about physical traits, to help me keep track of all those pesky little details, and then there is a section on family and childhood, your character's character (alright, that's a little cheesy I suppose, lol), likes and style, where and how your character lives now, life before the story, and then some cobbled together bizarre questions from various places.
I have found it very useful. Each section has 5-10 questions, and I find that rather than limiting my characters in any 2-D or stereotypical way, it has really helped me to polish my characters much more quickly than previously. Before I would have a plot half way finished and outlined, and the characters fuzzily formed, and then I would write. Somewhere during chapter 4 I would suddenly be struck with a problem about the characters history that I hadn't fully developed, and have to go back and change it... which meant editing rather than writing until it was fixed.
I also feel my characters are a little more complete, for lack of a better word. Maybe I am maturing as a writer, but I feel that it would take me some time, previously, to fully understand or flesh out some of the deeper dimensions of my characters. This time I feel like I have found all that before I have even begun the WIP, specifically because of this trait sheet. It has made me follow the natural progression and root causes of a lot of their personality traits. It's hard to verbalize (or the writing equivalent), but I guess all those aspects and traits I wish I had understood motivated my characters and the shaping of the story, I understand before chapter 7 and it comes as a surprise, and at the expense of depth for those first chapters.
Who knows....I might get to chapter 7 and think, "Gee! I didn't really know anything at all. MC1 just shot MC2 and I NEVER would have guessed that on her character sheet." lol.