I'm and Emerald Club member and member of several other rental car agencies. Of late, I have been doing Priceline.com for rental cars and have had great success.
My process is to reserve through one of the big companies (National, Avis. Hertz, Dollar, etc.) so I have a car locked in at the lowest published fare using coupons, membership perks, etc. These reservations can be cancelled till last minute.
I then go to Priceline.com as I get nearer to the actual trip and name my own price. I have not paid more than $15/day for mid or full size cars in more than a couple years on personal travel. Be aware that once your bid is accepted, you are booked and there are no cancelations. You also don't know which company you will get until your bid is accepted. But, so far, I've had only National and Hertz accept my bids and the end result is always savings of at least $100/week. Usually more. That results in extra spending money. I just do not start bidding until I am close enough that I am pretty sure the trip is really going to happen, so not to have to worry about cancellation. Once I get accepted at Priceline at MY PRICE, I cancel my other reservation made directly with the other agency. You do have to stand in line to get your car, no Emerald Aisle or frequent flier miles, but for the savings, it is worth it to me. This may not be for everyone. It took me a while to try it, but now, for personal travel, I use it every time.
BigTom