if you have a costco card then go to
www.costco.com and hit the travel section. You can usually get rentals for 25% off with an additional 25$ off as well. You don't have to be from the US to use this either.
We use this all the time, in fact were using it in LA in 3 weeks.
Also, in the 4 years i have used these discounts, i have never had to produce my costco card. At Alamo you don't even need to go up to the counter, just use the kiosk and get your car and leave.