Mousesavers has a good page on discounts which I have used in the past and I second the Costco suggestion also. I've also had some good luck with Hotwire. I'm tracking a 2 week FL trip in Sept out of Tampa (saved over $350 on airfare over Orlando, enough to cover the car rental for the 2 weeks) and Hotwire is currently $357 all in, while the cheapest conventional with or without coupons is around $475.