For Alamo car reservations thru 12/31/10:
Use coupon code AD8Z59SDZ - $20 off for a 3 day or more rental, expires 12/31/10, any size car Costco coupon code. Use this in conjunction with the SWA 7014923 Corp ID code to get additional discount rate along with 1.5 SWA RR credits.
Hotwire and Priceline are good but their rates are not refundable so you have to be sure of your plans. I will use them once I am definite on my travel and they are much cheaper than using the Alamo/SWA deals. The Hotwire and Priceline fees, being unrefundable and no SWA RR credits can cause them to be not worth it for me.