Jeans and T-shirts are hardly haute couture, so they can't be hard to produce and sell, right? Delivering basics may be more challenging than you think. Just ask the executives at Gap ( symbol), which oversees the once-popular, now tarnished Old Navy brand. Clothing retailers have been hit hard by the slowdown in consumer spending. For most people, clothes are discretionary items; fashion comes second to paying your mortgage or health-insurance premiums. But Old Navy's lesson applies to any industry: If you veer too far from your mission, you risk losing loyal, long-time customers. The key is to figure out how to expand your offerings without diluting your brand. When the first stores debuted 15 years ago, the chain emphasized low prices. It became the place to go for cheap basics. That allowed the previously low-key Gap brand to add more sophisticated items at higher prices. The stores were marketed as welcoming and inclusive; places where women's, men's and children's clothing were stocked side by side. The chain became a family destination that could attract a suburban mom, her teenage son and tween daughter. They could buy their own versions of the same fleece pullover or flip-flops. Last year, Old Navy started going after trendy, young shoppers with mini slip dresses and tiny bikinis. The chain's TV commercials went from peppy, '50s-inspired spots featuring retro celebs such as Morgan Fairchild to ones with slinky models.