Deals can also be found by upgrading. With demand for fuel-efficient cars particularly high, travelers can sometimes find a better deal by booking larger, roomier vehicles. A recent Expedia search, for example, found a full-size van for $90.42 a day at the Budget Rent a Car in Long Beach, Calif., compared with $97.86 for a standard car. While the added fuel costs (not to mention larger carbon footprint) might erase the savings, the extra space and comfort might prevent World War III between siblings in the back seat.

Also, look beyond national chains like Avis or Hertz, to the hundreds of independent car rental agencies. Because of lower operating costs and smaller overhead, mom-and-pop agencies, which can be found at sites like CarRentals.com and CarRentalExpress.com, typically offer rates between 15 and 30 percents less than national agencies.

Willing to gamble? Consider Web sites like Priceline.com and Hotwire.com, which offer deep discounts to travelers willing to be locked into a preset price before finding out the rental car company.

Another option: virtual coupons. Sites like FatWallet.com and CouponWinner.com list discount codes for car rentals, or type in the name of a car rental company and “coupon code” into Google to see what turns up. Good deals also show up on airline Web sites under mileage partner offers. For example, Delta is offering up to 20 percent off with the discount code CDP 165385, and double miles on Hertz rentals in the United States (including Puerto Rico) and Canada. The code brought the weekly rental of a Toyota Prius from Newark Airport in mid-July down to $503.90 from $685.05  a 26 percent saving.

If you don’t have the time to seek out such discounts, Steve Ellis can do it for you. After his wife pointed out his uncanny knack for finding rental deals, Mr. Ellis, who is a business adviser and frequent traveler, created RentalCarMagic.com to allow customers to pay for his deal-sniffing services. The site, which charges $14.95 to $49.95, sends back a quote in a day or two.

Chris McGinnis, editor of The Ticket, a subscription newsletter for frequent travelers, recently tried RentalCarMagic.com for a trip to Hawaii. After paying a $30 fee, he said, the service saved him $54 on a convertible from Alamo Rent a Car. Though he found the process slightly cumbersome  he still had to go to the rental company’s Web site to book the deal using the discount code provided by Rental Car Magic  he noted in his newsletter that in the end, “it saved us more than we were able to save ourselves.”