I own a car but but have to leave San Francisco Bay Area for a couple of months or so. The thing is I haven't rented anything so far, so just looking for a long term-parking with a possibility of prolongation. Currently the only option of this kind I'm aware of is parking near the SF airport. But it would be nice to consider all the possibilities.
There are numerous self storage facilities in that area, check out the yellow pages or do online searches. Many have outdoor parking available at reasonable rates ($50 to $100 per month). Some have covered parking as well at a higher rate.
They are mostly month to month rentals, no long term contracts, automatic billing to your credit card each month while you store there. When you get back, simply go get your car and check out of your assigned spot. We store our company vans and trailers at several locations around the states this way.
If you plan a long trip and have a nice new car, you can always put a tarp over your car to protect the paint from excessive sunshine.