Save yourself some money...use priceline..it is simple.
Look at hotwire to get an idea of the rates. I did a trial run and offered a rediculously low price, knowing it would decline. But it gave me the opportunity to get familair with the site. I would bid about 40% less than Hotwire to start.You have to wait 72 hours to rebid. I got a rental in SFO that would cost me around $150 on hotwire, and got it for $113. They use the same rental agencies. I would do priceline again for a car rental.