Get down payment assistance from San Francisco's Mayor's Office of Housing. This is for REO's and Short sales only, along with income restrictions, and quite a few other restrictions. But if you qualify, you can get help with up to $60,000 in down payment. Read up on "DALP" here.
There's also a long list of lenders who are "participating lenders" and I know two of them, Christine Rocca at BofA and Gordon Friedman at Guarantee Mortgage. So if you think you might qualify, give one of them a call for more details.