Taxis and shuttle services are readily available just outside the terminal.
We chose the first one we saw by the curbside, a red van proclaiming itself "Lorrie's Shuttle." There were already two other couples (four passengers) inside, so we were delegated to the last row. Tip: For convenience and if you are not as excited as we were to get going on our sightseeing, wait/look for another shuttle where you can get better seats and/or have better air-conditioning.
Fee from the airport to our hotel, The Prescott (right smack in downtown SF by Union Square), was $15/person.
Upon payment, you are handed a coupon, which you can redeem for a discounted rate/fare on your next trip back to the airport.
It is advised that you call 24 hours ahead of your scheduled return pick-up.