Sign up for emerald club, which is free, and you're always guaranteed a car as long as you book with your member ID,they make sure your car is available the night before.
Be prepared to be disappointed. This office is incredibly Inefficient and ineffective. It's like they don't know how to use a computer. I've consistently had a problem with getting a car here.
Get last minute deals on car rental rates for business and vacation travel at San Francisco Convention Center with National Car Rental - Bypass the Counter at the airport with the Emerald Club.