For virtually every segment of the fashion industry -- from Parisian luxury houses to small-fry entrepreneurs selling one-off clothes over the Internet -- the challenge is how to capture what marketers call ''mind share,'' that part of consumer consciousness not staked out by other influences.  Some companies, like Asprey and Louis Vuitton, do it with aspirational advertising and monolithic real estate, which by its very size works to establish a presence in peoples' minds. Louis Vuitton's 20,000-square-foot Fifth Avenue store, its opening marked by lines of disgruntled guests stuck out in the cold and a heavy police presence, is its largest store -- that is, until an even larger Vuitton opens in December in Paris.  Fashion is one of several industries facing its own perishability. The music and film industries have experienced a similar blight as CD's and movies have shorter and shorter shelf lives. But to sense the change in fashion one need only look at the runways in Bryant Park in New York. Last week, 130 designers presented their fall 2004 collections, with more to follow over the next month in Milan and Paris. Yet by the time their clothes are manufactured, a factory in Malaysia or China will have produced cheaper versions. Stores like H&M and Zara will have them on their racks -- and the consumer will be on to the next thing.  Even for an industry that survives on change, this sense that nothing lasts for long constitutes a radical shift, affecting moods and economies but also lives. No one understands the paradigm of impermanency better than Tom Ford, whose career at Gucci was abruptly shortened. Asked recently if he thought people would remember him after he leaves Gucci in April, Mr. Ford said to Women's Wear Daily, with a snap of his fingers: ''They will forget. This is today. This is the world. Six months, a year, two years. Whatever happens goes away.''  For years, corporations have recognized that ''the medium is the message.'' But as the sheer glut of information clogs up the sensory canals, making traditional news media less effective -- Ms. Koehn estimated that American consumers w are bombarded with 750,000 individual advertisements a year -- marketing experts say how a product is sold can break through the clutter. ''What you'll see is that distribution will become the message,'' Mr. Matlins said.  ''The young people, the people in the pit of fashion: they're turning in a different direction,'' said Mr. Joffe, who added that the hyped-up atmosphere of fashion -- the megashows, the stars -- has led people to seek a more ''authentic'' experience. ''Big brands mean less and less to them.''  Melissa Kirgan, 23, who graduated last year from the Fashion Institute of Technology, gave up a good job with a sportswear company to form a collective of designers and artists, one of several such groups in Manhattan. ''Our approach is anti-Wal-Mart,'' she said. ''It's turning against all the branding and noise.''  Perhaps no retail concept is more reflective of these provisional times than the new Comme des Garçons Guerrilla Store.  Mr. Joffe said that when he and Ms. Kawakubo visited Berlin last summer, they were impressed by the authentic-looking atmosphere of the Mitte district, especially at Monsieur Vuong, a Vietnamese noodle bar that draws a mix of old and young people, and at Elternhaus, a boutique run by a group of artists whose name, in German, means ''parent's home.'' None of those places seem to devote a great deal of attention to décor or displays. Unconsciously or not, their owners were recognizing a fundamental shift in young consumers' attitudes: that content and product now counts for more than image.  By early October, Ms. Kawakubo and Mr. Joffe had formulated the idea for Guerrilla Store, enlisting Dat Vuong, the owner of the noodle bar, to help with décor. They also spread out their risk, creating a partnership with Christian Weinecke, an architecture student, who will run the shop.  Because Comme des Garçons is a small company, with annual sales of $120 million, it isn't encumbered with layers of bureaucracy and can respond to changes quickly.  The Guerrilla Store also makes commercial sense. With a monthly rent of $700, the store gives Comme des Garçons an inexpensive means to channel avant-garde pieces from the runway, sell off clothes from past seasons and reduce inventory.  Still, even if companies like Gucci and Louis Vuitton are too invested in their luxury images to be sincere guerrillas, marketing experts say they need to be much more attuned to the fact that young people, whether in New York or Shanghai, are tapping into the same underground. And that can make a designer wallet seem a lot less desirable than a customized pair of jeans.  ''The influences today are terrifically eclectic,'' Mr. Matlins said. ''One of the things we're seeing with kids is how much they're buying from eBay.'' When Sidney Toledano, the chief executive of Dior, visits New York, he makes a point of stopping in the Apple computer store.  ''I learn more from a store like that than I do fashion stores,'' he said. 'You're not just buying merchandising, you're exchanging information.''  Berlin, with its long history of occupation, may be the perfect place to start a revolt. Among the 200 Berliners who passed through the store searching through cotton shirts and boiled wool jackets was Claudia Skoda, a knitwear designer who used to have a shop in New York.  ''It seems a little bit funny,'' Ms. Skoda said of the notion of a store not meant to last forever. Still, she bought one of Ms. Kawakubo's orange checked skirts and planned to return for a pair of silver sneakers. She said she already knew of a designer who was thinking of opening a similar store in Mitte. And she was pleased to find something different in her own city. ''I think people are tired of things you can get everywhere in the world.''
New Comme des Garcons Guerrilla Store in Berlin is set up at little cost in former bookstore, with minimal advertising, as transient space to lure fickle shoppers for short time; Tokyo-based company set up by Adrian Joffe and Rei Kawakubo plans to open twenty such stores by next year, including one in Brooklyn; stores will stay open one year, acknowledging highly perishable nature of fashion and reflecting consumers' wish for different shopping experience; Target's recent 'pop up' Mizrahi outlet in Rockefeller Center last fall is also example of trend; photos; marketing strategy discussed (M)