Both working together actually. I have an idea in my head of what I need to fill a wardrobe hole, then I start looking around to find something that fits the bill. It can take a short time (Tulle jean jacket or Etsy jewelry) or considerably longer (some things have been on my list for a long time and still are).
Ex.1: Tulle jean jacket -- I decided I finally wanted to get one to wear with "girly" dresses but it had to be different from the typical western looking ones you see. I happened upon Fashion for Nerds blog and that day Audi posted the Tulle jean jacket I had to have. Searched on ebay and bought it that evening.
http://cgi.ebay.ca/ws/eBayISAP.....K:MEWNX:IT
Ex.2: cowboy boots -- have been on my "list" for years now, but they are expensive for how much I wear them. In my city, the majority has cowboy boots in their closets due to the annual Stampede Rodeo and Exhibition. I happened on a pair (dark navy ropers) that fit perfectly in a thrift store the other day -- my size, right before the Stampede -- very rare here!
Ex.3: Etsy jewelry -- did a search on Etsy for ruby jewelry to go with a dress I am wearing to a wedding in August. I like vintage styles for dress up but I don't really wear jewelry in everyday life. I was willing to settle for something not exactly my taste just to get something. I found IMO the perfect necklace and earrings set.
http://www.etsy.com/listing/74.....brass-bead
I have been influenced by YLF too, so I think I am more open to things that previously I wouldn't consider.
I do a lot of thrift shopping so most of those buys are "lucky" but I still have a preconceived notion of what I am after, even if only a slight idea.