As a Realtor and real estate trainer, I do my fair share of travel and am always trying to find the least expensive option that is still viable. One thing that frustrates me is rental cars since the exact same product is priced so differently depending on how you shop.
I used to call (or log into) all the rental companies and start comparing the variety of discounts they offer to see which one is most beneficial for me. Then, after I thought I did my research, I would even try using a site like Priceline to see if I could do even better.
However, now I have found something that works amazingly well and results in the use of one website and no coupons. My favorite solution now is www.CostcoTravel.com
They apply their bulk pricing, then add in coupons they are aware of to come up with their pricing. They show rates by vehicle type and company and let you decide which is best.
However, one more thing you might check on your trips is how much cheaper it might be and if it makes sense to pick up the car at another location besides the airport since many airports include additional fees that can be avoided by going just a few miles away. Here is the difference for the same time period but not picking up and dropping off at the airport.
I researched these rates for October 1 - 4th for the Idaho Association of Realtors conference and the Idaho CRS Chapter. They are always subject to change, but it is still a good sample of what you might see.