On the #KX, you're allowed only one carry-on bag; on the #292, you can bring as much as you want provided you can carry it onto the bus yourself. The SFO Airporter bus  picks up outside each baggage claim area every fifteen minutes and travels to Union Square and the Financial District. The blue SuperShuttle, American Airporter Shuttle and Bay Shuttle minibuses depart every five minutes from the lower level of the circular road and take passengers to any city-center destination.

Be ruthless - competition for these and the several other companies running shuttle service is fierce and queues nonexistent. Taxis from the airport cost for any downtown location, more for East Bay and Marin County - definitely worth it if there is more than one of you. If you're planning to drive, the usual car rental agencies operate free shuttle buses to their depots, leaving every 15 minutes from the upper level, but you have to load and unload your bags yourself. The car rental depot is located north of SFO and is too far to walk to.

Several domestic airlines (Jet Blue and Southwest are two) and United fly into Oakland International Airport (OAK), across the bay. This is actually closer to downtown San Francisco than SFO, and is efficiently connected with the city by the AirBART shuttle bus, which drops you at the Coliseum BART station. Get on BART and San Francisco's downtown stops are fifteen minutes away.