The Gossip Girl phenomenon — which mirrors the impact Sex and the City had on high-end footwear sales — shows how retailers can capitalize on trends in popular culture. It also illustrates how vital it is for them to quickly seize on the hottest clothes and accessories, given today's impressionable young shoppers, and to closely track sales to avoid shortages of the latest fashion passion.

Trend watchers at retailers including Target and J.C. Penney monitor fashion-forward people in cities around the globe. Increasingly, though, retailers also need to watch a lot of TV and movies and read stacks of celebrity magazines, lest they miss out on the Next Big Thing.

As recently as a couple of years ago, trend spotting often had to be done a year in advance — for next summer's seasons, say, even before the current summer's merchandise had been sold. Penney and other retailers were forced to make "educated guesses," as President Ken Hicks calls them, based more on speculation than hard data.