If you end up parking downtown, double and triple check to make sure you didn't park in a tow-away zone. If there's a sign anywhere on your side the block that says tow-away, even all the way on the other corner, be prepared to run back to your car or find a different spot. Streets like Geary have red topped meters, making it quite hard to overlook. Other streets are not so obvious, so watch out!
The majority of meters downtown are also 2 hour only. That means it maxes out at 2 hours when you fill it. Then it's just a matter of going back to the meter and filling it up again, which is supposedly a violation, but I don't think anybody's been given a ticket for feeding the meter.
Be very quick when parallel parking, a lot of drivers here won't hesitate to swoop in behind you and steal the spot.
I wouldn't leave my car parked overnight anywhere near the Tenderloin, I've seen plenty of smashed windows to know better.
Expanding on the don't park in driveways idea, pay close attention to the lines when parking downtown. On a lot of the spots there will be a 'T', with the top facing the sidewalk. The top tells you how far out you can be, and the line defines your spot. If you find a spot that you can fit in, but requires you to even be on the line, pass it. Meter maids will give you a ticket even if your bumper is on the line.
Don't park more than 8 inches away from the curb, ideally you should park within two inches.
WATCH OUT FOR BICYCLISTS!!!!!!!!! This is no joke, anyone who lives in SF knows how a cyclist can pop out of nowhere with the idea that they can overtake you. Not a lot of them follow the majority of traffic laws; so if you pull out of a parking space while one is coming up, causing them to swerve out of your way, you can get moving violation.
If you're going to be driving around in SF please please please do not be a timid driver, and obey stop signs. I've never been close to being in an accident with another car, except when people think stop signs are optional and blow right through them. Slow down when you need to, speed up when it's okay to. Don't hesitate to take your right of way, and you won't have me screaming profanities at you while revving my engine.