Methodology: Compare Expedia, Hotwire, Kayak and Orbitz.  Then check the web site of the lowest priced offering to see if the price is even lower. 

Recommendations: Use Hotwire and Kayak.  Together they had the lowest prices 100% of the time.  Note that Hotwire is non-refundable and does not award frequent rental points.  Hotwire only uses Avis, Budget and Hertz so it's tough to go wrong with them. We like Kayak because it finds excellent prices and has fast & easy filtering of results, just un-check the car types that you do not want.

We recommend joining frequent renter programs of Alamo, Budget, Dollar, Enterprise and Thrifty (the usual low cost winners) to eliminate waiting in line at the counter and the pressure to upgrade, purchase insurance or gas.

Check any special pricing that has been negotiated by your employer or association.  You may have discount codes that can lower the price (AAA, AARP, etc.).  Some codes are mentioned at various discussion threads at FlyerTalk and BiddingForTravel.  However, the code that produces the lowest price in Boston may produce a higher price elsewhere.  Most coupons that are included in your airline's frequent flyer mailing and credit card statements require a multi-day rental that usually includes Saturday.  From our limited testing with various coupons and promotional codes, they did not provide a lower price.

Lower prices may be available at Priceline but you must do your homework first.  Find the lowest price as suggested above and then visit BiddingForTravel for excellent advice.

A significant amount (20% to 50%) of the total cost is due to taxes and fees imposed by governments and airport authorities.  Remember: visitors can't vote! We have been told that you can avoid these fees and significantly lower your costs if you arrange to get to the rental car lot outside the airport without using their shuttle.  This technique has greater benefit for longer rentals.

When renting in Europe or the UK, check the wholesalers that are listed in our "Travel Links" web sites section.

Most sites have some frustrating limitations requiring extra steps:

Don't get caught:  Some sites promote their sponsored or preferred vendors by placing their cars at the top of the search results even if lower prices are available from other suppliers.  You must then find the button or link that displays all of the choices or re-sorts the results into the lowest cost sequence.  Failure to notice this can cause you to pay more than necessary.  This is deceptive and a waste of your time.

Migration of cars: Florida needs rental cars for tourists from late November through April.  The rest of the year, cars are needed in the northern states.  The major rental companies offer significant discounts if you pick-up a car in a northern state and drop it in off in Florida in the fall or pick it up in Florida and drop it in a northern state in the spring.  The discounts allow you to have a car at a very low daily rate.  The prices for moving a car are not posted until very near the time they are needed, so sign-up for the newsletters from the major rental companies such as Avis, Budget, Hertz and National.  When combined with a one-way ticket from a low-fare airline, it's a great way to save and have a car to drive during vacation.

Finding low priced gas: If you are a member of Costco, use their "locations" button to view stores along your route.  Also check GasPriceWatch, the site has a "My Route" tab that is helpful.