We use DH's company's discount, he works for a big corp. So you might want to check with your HR department to see if there are any perks. There are lots of perks on the HR website that we use. The rental car offer is on the 2 or 3 companies, so we just do the homework to get the best rate. We have found in some instance's that picking up your car at the airport is a lot more than from a regular office. So a lot of times I will check the local offices and get a quote, and then call the airline option and see if they will honor the local price, its been 50/50 so far... if they wont match we go with the local office that offers pick up at the hotel, then a ride to the airport after drop off.
In Vegas we rented a car ( SUV for 6 adults) for the day at one of the hotels, it was 59.99 for the day, but after the discount we got it for 39.99 unlimited miles.
Also calling and talking to a real person has had some luck in getting us a better rate. When you point and click is not always the most cost effective way to go, again 50/50