Do a bit of reading at
http://biddingfortravel.com/ and then Priceline it.
I usually hit up Orbitz, Expedia, and Travelocity as well to see what deals they may have. Then I go to the car rental Web sites of the lowest price, name-brand car rental places and see if they can do better.
I advise that you avoid the no-name car rental places (Fox and Payless car rentals come to mind). They will do everything in their power to screw you once you show up at their counter, and then they'll give you a car with 300K miles on it (no joke!).
If you do decide to go with one of these el-cheapo places, make sure to bring a copy of your insurance policy and rent only during your insurers business hours. This advise is based on my own experience at SFO and my sister's experience (despite my warning) at LAX.
Good luck.