I reserved my car through the USAA website (for USAA members only) and got a better discount than any other discount code that I've seen yet. Plus I was upgraded to a Premium car (small print says if one is available). Basically I got a Premium car for less than the cost of an Economy car (using the AA code and the Costco code).

This may not work for everyone, but it's a great deal for USAA members.

Nick