Here was my response:
Are you brand loyal – meaning you will only rent at one company? If so, it may be a little harder to find deals. Price the competition using www.kayak.com. Then call the manager of the location where you usually rent and ask them to match the rate. Non-airport locations may be more likely to do this.
Kayak.com is a search engine that pulls together all the online prices from the company’s website. It will also pull prices from travel consolidators like Travelocity or Expedia if you click those boxes. This provides a good overview of the going rate at any given point in time.
If you are interested in trying other companies, take the prices from Kayak and compare those to what you can get through Southwest.com. On Southwest's main page, they usually have ads for rental cars at 30% off. Click those. You don't need to have a flight.
I have had good luck on Priceline with the name your own price function for rental cars in large cities. You don't see what company it is until after you pay and it's non-refundable. I usually can get a higher class of car for the price of an economy.
Have you found great deals for renting cars? What do you do?