"She's a beloved American icon," says Flickinger, adding that the merchandise in the line has remained popular because they're "timeless, in good taste and have quality."

Kathy Ireland, 49, a former Sportswear Illustrated swimsuit model, also turned her celebrity brand into a moneymaker. Since 1993, she has built a $2 billion global retail business, according to fashion trade publication Women's Wear Daily. Her line includes more than 15,000 items from curtains to wedding dresses that are sold in more than 50,000 small chains.

Ireland attributes her success to her methodical approach to expansion. In fact, her first foray into the business was socks. She wanted to see how something simple would sell before she rolled out swimwear, active wear and other items a year later in 1994.

"If women would embrace something as basic as a pair of socks, that would tell us we were on to something," says Ireland, who sketches looks for her line for a design team to refine.

More recently, singer Jessica Simpson, 32, has built her brand into a billion-dollar brand in the past seven years. She now sells more than 29 products from shoes, clothes and perfume to purses and luggage in department stores such as Macy's. For the winter holidays, items include $89 platform bright blue platform pumps and $128 strapless belted lace dresses.

Her formula for success has been having a relatable personality: Even as her singing career has wavered, branding experts say Simpson has been able to connect with her young fans because she's vocal about everyday issues like her struggles with weight gain.

Peggy Merck, the publicist for the brand, also says she's very involved in designs for the line, which reflect her casual but sexy style. Her collection, which ranges from size 2 to 16, features lots of cowboy boots, vintage jeans and wedge shoes. Simpson is "hands on," Merck says.

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