TRI Coldwell Banker, SF's #1 Office
Have your realtor run some comparables for you and make sure you get a CMA cover page which lists Active, Pending & Sold. The Sold units are the key. Then go to the Assessor's Office in person and file the paperwork to request lowering of your tax bill. Be sure to keep a record of the date, person you spoke with and what they said verbally to you.
If you do not get a letter back in response, call them. Be sure to give them time to reply as I am sure they are overloaded since prices have fallen. But I would make my first effort face-to-face. You will get a better and faster response.
The crieria needed to lower your assesed value is available at the assessors office at City Hall. First remember that the tax is 1.14% ($1,140 on $100,000 drop in valuation) so there is a point where going through the work getting it redone isn't worth it.
If you have comparable sales showing not much of a difference it might not be worth it. Much of San Francisco hasn't gone down in value as much as other areas in the state ofr Bay Area.
Check out the ramifications also. I've heard that if yiou do get a lowered valuation the assessor will re-examine the property in a few years which might put you higher than you were at sale.