A sluggish economy has offered an opportunity for the little guys to wield considerable power. Occupy Wall Street, take note.

By Anne Fisher, contributor

Tim Tebow: Underdog of 2011?

FORTUNE -- Perhaps because we feel powerless ourselves, rooting for the underdog, especially in tough times, seems to come naturally. The tendency is even ingrained in popular culture, says Amy Showalter. "In how many movies does the big, powerful -- and usually evil -- guy win?" she asks. "Not many. That's partly because the underdog is perceived as more moral, more resourceful, and more deserving of success."

Still, not all underdogs are created equal. "Just being at an obvious disadvantage won't guarantee that people will want to see you win," she notes. Showalter is president of the Showalter Group, a lobbying firm that has organized grassroots political efforts for more than 150 companies, including Southwest Airlines (LUV) and Pfizer (PFE).

For her new book, The Underdog Edge: How Ordinary People Change the Minds of the Powerful…and Live to Tell About It, she analyzed heaps of academic research and dozens of real-life examples to figure out what gives some underdogs what Showalter calls "extreme influence." Determination, a vivid story of struggling against adversity, and the willingness to try harder than your competition all help. So does standing out from the crowd by stubbornly insisting on doing what the public perceives as right.

You don't have to be small to get "street cred" as a David against the Goliaths, Showalter says. Consider, for instance, Southwest Airlines. It's now the largest domestic U.S. carrier, but the company "has always played by the underdog creed, most recently with its 'Bags Fly Free' initiative," Showalter writes. "Southwest lets passengers check two pieces of luggage for free, while others charge $20 or more to check one bag." Southwest CEO Gary Kelly credits this one underdog move with boosting the airline's annual revenues by about $100 million. More