The key to creating a positioning advantage is to differentiate yourself in your customers' minds in at least two dimensions and hold positions in those dimensions that your competition will not want to occupy.
Example: Urban Outfitters, a fast-growing U.S. clothing retailer, markets only college-age students. Traditional competitors, like Gap, will resist copying this positioning because to do so would be to give up other important market segments, like baby, maternity, and, generally, adults. Urban Outfitters also differentiates itself by making every store look a little different. Gap, who depends on efficiencies gained by every store looking similar, will resist adopting Urban Outfitters's layout strategy. Of course, traditional competitors can replicate this "college student-focused and every store unique" positioning with a new brand, a new concept, or a new area in each store, but that takes years of planning. While competitors plan, Urban Outfitters grows.