Best bet is to alternate between craigslist & walking around the neighborhoods your interested in and looking for for-rent signs. The around SOMA/South Beach are very nice though a little lifeless - they tend to be very overpriced but then offer as much as 4 months free with a 12 month lease!! (Be sure to negotiate) Older buildings still will generally give you a free month; most small landlords won't but will negotiate on the rent; you should be able to knock off at least $100.

First thing to do though is pick which neighborhood you want to live in; San Francisco often feels like a dozen different cities where you can walk from block to block and the character dramatically changes.

-Jay