Union Square is
the place for shopping in San Francisco--if you have plenty of cash, that is. Around the square are several big name shops like Saks, Macys, and a huge Borders bookshop with several floors (you will be tempted). The Levi flagship store is here too, and it's pretty cool--you can pick out a pair of jeans and take them down to the ground floor to have them personalised or altered; if you have time, you can also order Levis to your specifications (style, material, and measurements).
In the square itself are several arts-and-craft stalls, especially on weekends. On a sunny day it's a good place to sit down with a coffee and people watch.
The shops continue on from Union Square, and you'll find a Virgin Megastore, FAO Schwartz, Chanel, Sephora, etc. If you walk towards the Civic Center though, the shops get distinctly seedier, while expensive shops and boutiques continue in the opposite direction. (There is a little market selling cheap sunglasses and so forth by the Civic Centre).
Across the road from the Cable Car turntable is the main shopping mall: seven floors of shops with Nordstroms department store occupying the top four. The mall consists mostly of brandname clothing shops like Guess and Victoria's Secret, and is not the place to get a bargain.
Haight-Ashbury shops are a world away from Union Square (although the neighborhood does have a Gap) and you can definitely find a bargain here. The shops range from retro second-hand and tie-dyed sarongs, to the latest urban fashion, with a biker's shop, old-fashioned cobblers, and alternative bookstores in between.
The other place to try for cheaper options is the Mission, San Francisco's Mexican area, which also sells the expected leather and silver goods.