Also look into Costco rates, even if you're not a member. You can pull the Corporate code and the Coupon code off the the Costco website and use it at the car rental place's main website.
I am a Costco member, and have used those codes, but have never been asked at the counter to show my Costco card.
AAA discounts are also available if you are a member.
It is impossible to create an idiot-proof product. Humanity is simply too adept at churning out better idiots.