cab it
Even though public transportation is sufficient in downtown SF, sometimes one just need a taxi to get some where in a hurry.
SF does not have the best taxi service, but one rule of thumb to get taxi is to go to or near hotels. Some of the major ones that cabs circle are Hilton by Embarcader, Westin St. Francis in Union Square, Marriott on Fouth by Mission. Even if you are not at the taxi stand, there will be cabs driving by.
Generally, calling a cab in SF is not too reliable. You are better off with a shuttle service to the airport if you need one. Cabs are great for getting you around the downtown area if you are in a hurry.
