I have used CarRentalSavers.com with decent results in the past. It's basically a meta-search-engine that apparently knows about all the current valid coupon codes for each rental company. The one bad experience I had with them was trying to book a one-way rental once; the rate they quoted me said it included unlimited mileage, but when I double-checked with the actual rental car company, it turns out it didn't -- and that would have been a massive extra charge had I not caught it ahead of time. But for standard roundtrip rentals I think they're fine.
My biggest "hack" or tip or renting cars, especially from airports, is not particularly a financial one, but a time-saving one: if you sign up for the company's free frequent traveler/rewards program even if you don't intend to become a frequent renter of theirs -- and then use your ID number when you book the reservation -- that often allows you to skip the reservation desk in the terminal (where there are always long lines and minimal staff) and just go straight to where the cars are. In many cases, you will find they already have a car pulled around waiting for you, or in the case of National (my current favorite when I go on work trips) you get to pick whatever car you want out of a certain row and just hop in and go. Saves a ton of time.