Since their garment factories are located in Southern California, American Apparel can response to the market fast. They can get the products that are selling in stores right away.
Also, it makes them easier to keep high quality and inventory control- no markdown and less sales lost to stock outs.
American Apparel makes basic items which have no logos on and less graphics so that they don't have to Forecast fashion trends. Instead they make various color range that brings them a strong brand image.
When I think about American Apparel besides their clothing, their unique advertisement campaign comes up my mind. They don't use models for ads, but familiar images of employees and friends from around the world. It allows them to convey diversity and differ from other luxury brands.