Unlike fighting for a cab in Manhattan, San Francisco can be a desert in needing a taxi at certain times and certain corners. Outside of the Tenderloin, Union Square, or Financial District, there are no cabs visible as far as the eye can see. The trick about getting a cab in SF is to know the taxi patterns: when and where cabs are empty, after a nearby drop off, and to stand in a favorable direction for line of sight, and in favor of the direction in which you plan to go.
Market and Valencia is a great place to get in an empty cab to travel up Franklin to Russian Hill or Fort Mason. Eighteen and Castro is great in the evening to go home anywhere in the city. Major 4 star hotels almost always have a queue of cabs, and if they do not, then you know you are in for a wait to nab one.
The key here is to know where a nearby cab will be dropping someone off. Look for the smokers standing in front of a club! A cab will pull up shortly. The important thing to note is that you must move to an area where people are standing on the sidewalk, or where there is heavy pedestrian cross traffic.
Hey Driver Doug, have you heard about Uber and Lyft? It’s time to get a smartphone! Get the app!*
*Author’s response (2022): I got my first iPhone at Target in 2017. I now have an iPhone13 mini, and am finally current.
The Dao of Doug: A Bus Driver's Perspective 