Simply put, negotiate. For our vacation to Myrtle Beach, we are renting a car. Originally, I had rented a car through Avis using my corporate discount (AAA and USAA offered discounts; however, they were not as generous). Unfortunately, picking up the car was going to be a hassle.
Although Hertz's online rates (even with AAA and USAA) were not as good as Avis, I called the Hertz branch (which is located in my office building) and asked them to match the Avis rate. Not only did they match the rate, they beat it by $50 and they will not charge extra for the 2nd driver.