Very often, when renting a car and dropping it off at a different location than where you picked it up, you have to pay a huge one-way fee.
Have you successfully used any technique, trick, or tactic to avoid paying such a one-way fee?
You didn't specify a locale, but within the United States, the answer depends a bit on when and where you are picking the car up and when and where you are dropping it off.
The obvious first step is to search as broadly as possible and to be as flexible as possible. Compare rates at Kayak, VroomVroomVroom, and other comparison sites.
Planning the trip
Sometimes, the length of the trip factors into the calculation. A few years ago, I rented a car in Binghamton, NY and drove up to Watertown for a 3-day trip. I then needed to drive to Washington, DC and was quoted something exhorbitant for the 4-day, 1-way rental. After talking with the agent over the phone, however, I "returned" the car to Binghamton on the way down and made a second rental— in fact, the identical vehicle; I never even took out my bags. In other words, instead of a 4-day 1-way trip, it was a 3-day return and a 1-day 1-way trip, and that saved me almost a third. Yet I have also heard anecdotally of the reverse, where a 2-day 1-way gets a $300 surcharge and the 3-day 1-way does not.
Enterprise and perhaps some other agencies only charge the one-way fee if the drop-off location is more than 500 miles from the pickup location. In still other cases, you may be exempt if renting from corporate-owned locations but not from franchises. You will need to do some research on your own whether the cost advantages of renting from a neighborhood location (namely avoiding airport surcharges and taxes) outweigh the disadvantage of paying the one-way charge.
The major rental agencies offer one-way deals as they try to redistribute cars around the country to meet demand. In particular, cars accumulate in warm-weather states in the winter, so in the springtime you'll often find huge discounts if you pick up in Florida or Arizona and drop off in the Northeast or Midwest (Hertz for several years has offered $5/day to take a compact from Arizona and drop it off in California, Colorado, or a few other states). Check the specials pages on their websites. Note that the deal usually applies to only certain types of vehicles and to certain locations, mostly airport locations in major cities.
For Australia, Canada, New Zealand, and the US, try searching on auto relocation agency sites like iMoova and TransferCar for current offerings.
Some agencies may not charge a one-way fee if the travel is between airport locations entirely within a few large states, such as California or Florida, where there is a healthy circulation of vehicles.
You are less likely to find such deals from neighborhood locations or from smaller agencies, because they have smaller fleets with higher utilization and are thus less "liquid" with their cars.
Fee-free rate codes
Some contract rates or coupons waive the drop-off fee, as various organizations negotiate special rates on behalf of their sales staff or other traveling employees. You can find many lists of such rate and coupon codes on the Internet; however, you may be asked to demonstrate that you qualify for the particular rate, and in some cases, you must present a physical coupon.
A driveaway is not a traditional rental car; instead, you are transporting a car on behalf of its owner from one location to another. In essence, you are a delivery service. Companies like AutoDriveaway can help set you up with a car. As might be expected, however, there are significant differences from a traditional rental car. On the one hand, you may get a newer, cleaner, or fancier car, as well as save a lot of money. On the other hand, you will face a high deposit, time and mileage limits, and restrictions such as no eating, no nighttime driving, or no using the trunk.
Finally, in some cases there is simply no possibility. I know of no agency that will be happy if you rent from Fairbanks, Alaska and return to Seattle, because there are so few cars that will be dropped off in Fairbanks. Specialty cars (sports cars, convertibles, etc.) also tend to be excluded from deals, because they do not circulate as freely and because their patrons are less price-sensitive.
International rentals are also tricky. I know anecdotally that trying to rent a car in Western Europe and return it in Eastern Europe is a daunting prospect. Laws can trip you up; for example, Canadians cannot drive a U.S.-registered rental over the border due to import laws (although they can rent a Canadian-registered vehicle in the U.S. and drive it cross-border).
Certain car rental companies have membership programs which have waving this fee as one of the benefits. For example, I frequently use National (in the US) renting in one airport and returning to another, using my Emerald Club membership to avoid the fee.