I really like Firecracker's analogy of food shopping. I think what makes it work is when you know what your tastes are (your style descriptors), you have the spices/staples you need at home (the wardrobe basics), and you have some FFB's in the kinds of dishes or outfits you make so you know what silhouettes will work, and probably have a limited color palette as well.
Getting to that point for me was a process where each new purchase revealed more to me about the direction I wanted to head and I had the best luck when I wore one item a few times before buying the next item to go with it -- I needed that time to 'live with' new purchases.
But it's true, when you have pinpointed an exact 'completer' piece it can be quite hard to find. Sometimes I just give up and wait until next season or year. Sometimes I find that something else fills the 'hole' in a different way than I anticipated.