Another way to find a great deal, especially at short notice, is if you're willing to book a rental without knowing the car company in advance. Two sites that offer deals of this kind are Hotwire and Priceline.

With Hotwire, once you enter your pick-up location and specify your vehicle type, Hotwire will offer you a complete price, including taxes and fees, though it won't show the breakdown between those taxes and fees. Also, you won't know your rental car company until you agree to book the car. Hotwire representatives recommend booking as soon as you see a price you like, as all quotes are based on availability and are subject to change.

If you want to name your own price for a car rental, Priceline is another type of bidding site that allows you—and not the rental company—to set the price.

To use Priceline, you have to be willing to book your car without knowing your rental company up front. If Priceline accepts your bid, you're locked into the rate. To make sure you don't over-bid, you should use the benchmarks listed in Step 2 of this guide, and bid about 20 to 30 percent lower. You'll know within a few minutes if Priceline has accepted your bid; if not, you can re-bid by changing one of your preferences, such as choosing a different pick-up location or car type. However, you can't make the same request for a car for another seven days.

Your total cost on Priceline is your accepted bid plus rental taxes and a processing charge per rental, so be sure you are comparing the total amount to see whether Priceline or Hotwire offers you a better deal.

One of the other main differences between Hotwire and Priceline is that you have more flexibility with Priceline in certain situations, such as if you need to book a one-way rental or rent from a non-airport location. But neither website will provide refunds or allow you to cancel booked rentals. In addition, you can't earn frequent flyer miles for your rental, or receive other benefits such as those from car rental loyalty programs. Also, neither website accepts coupons for upgrades, free days, discounts, or any other promotions.