IT'S HARD to work up any pity for someone bound on a summer holiday, but the truth is, life isn't all that easy for a traveler these days. When constantly changing airfares baffle even experienced ticket agents, it's no wonder many of the rest of us become bewildered facing the equally chaotic realm of rental car rates.

If you have your wallet at heart, finding the best bargain in a rental car that meets your needs (and then making sure the car is on hand as promised) can be a tedious, time-consuming chore. And even then, maybe the only thing that can be said for sure is that somebody else probably got a better deal.

On the road to the right price, the unsuspecting renter can tumble into any number of pitfalls.

One Washington traveler seeking a cheap rate for a week's rental in California was satisfied when a small firm quoted him a price of $135 for a mid-sized car. Only when he phoned back to check on the reservation did he learn, inadvertently, that the rate entitled him only to a two-door car. He had assumed midsized meant four doors, which is what he wanted. The cost of two more doors: an extra $10 for the week.

Not a big difference, of course, but when the man showed up at the rental counter making the switch could have led to delays, arguments and an unpleasant start to a vacation.

A couple arriving in Phoenix a few months ago got two surprises when they showed up with an overload of luggage for a three-week driving trip. Surprise No. 1 was that the rental outlet was 15 minutes from the airport; they had to look up the firm's phone number and call for a courtesy bus. That was a hassle they would rather have avoided.

Surprise No. 2 was that the firm carried no collision coverage on its vehicles. Customers were responsible for any damage up to the total value of the car (many firms make the customer liable only for the first $500 to $1,000). To avoid any damage liability in an accident, customers had the option of paying $6.50 a day for a collision damage waiver--a substantial sum for three weeks. If they chose not to pay the extra charge, they were required to leave a $300 deposit, in cash or with a credit card.

The couple figured their own auto insurance covered any car they rented, which is generally true, but they weren't sure. Frustrated, they canceled the reservation, arranged for a ride back to the airport and rented a car from a different firm at a higher rate. But with that rate, they were liable only for the first $500 in damages if they had an accident, and they didn't have to pay any additional collision damage waiver fee. That was a gamble they were more willing to take.

For more than an hour they had tromped through the heat with armloads of luggage, an aggravating beginning to a vacation. But it would be hard, in this case, to blame the rental firm. It had offered a cut-rate price that initially attracted this couple, but their travel agent had not warned them of the conditions attached. It's helpful to check what your auto insurance covers before arranging for a rental. You probably won't need to pay for the collision damage waiver.

Not that the rental car industry is without fault. The business is wildly competitive, and for the most part that is a plus as far as prices go, especially at such popular destinations as Florida and California. But the scramble for customers can lead to excesses. What sounds like a super deal in an ad could turn out to get you only a two-door mini-car, unsuitably tight-fitting for a family of four carrying a lot of camping gear.

"It's really a cutthroat business," says Steve Jones of the Council of Better Business Bureaus, which has been working with the industry to set advertising guidelines. Ron Smithies of the BBB's National Advertising Division points out a number of recurring problems in rental car advertising:

* Omitted facts: The price may sound good, but the ad doesn't mention that the firm charges a compulsory daily fee for the collision damage waiver. Or that you may not be permitted to drive the car out of state. Or that you must reserve a week in advance to get the budget rate.

* Faulty comparisons: Some firms have been known to compare their current prices against those of a competitor listed in the Official Airline Guide, which may be several weeks out of date.

* Small print: An ad may proclaim a standard rate nationwide, but then add in an obscure corner "at participating dealers only." Maybe a majority of dealers do participate, says Smithies, but they ought to be the dealers who are doing the biggest volume of business, which hasn't always been the case.

* Limited availability: One model car is featured at a special price, but there may be only a few of them on hand.

Being aware of these sometimes misleading practices can be helpful when you are trying to decide on what offer to patronize. But there are so many firms--from nationwide giants to small local companies working on a shoestring--seemingly all with different rates. How do you choose?

One day recently, five national firms quoted rates ranging from $135 to $199 to rent an intermediate-sized car (Buick Skylark, Ford Fairmont) at Los Angeles for seven days in mid-June. Only one--Hertz, the most expensive--guaranteed a four-door. With two, Budget and Sears, the reservations had to be made a week in advance. The cheapest, Alamo, was located away from the airport, with a courtesy pickup promised every 10 minutes. The drop-off charge to return the car in San Francisco ranged from nothing (Sears) to $100 (Thrifty).

In this sample, all of the firms offered unlimited mileage, but that is not always true. All of Hertz's corporately owned outlets (about 80 percent of the 2,000 nationwide) provide unlimited mileage. But the franchised Hertz outlet in Jackson Hole, Wyo., gateway to Yellowstone and the Grand Tetons, sets a limit of 700 miles in its $235-per-week rate for an intermediate-sized car. That's to keep the cars from being run into the ground in a region of vast distances, says a Hertz spokesman. Anything over 700 is charged at a rate of 33 cents a mile.

One way to sort things out is to talk to a travel agent who includes making car rental reservations as a free service in any business or vacation travel arrangements. Many deal daily with rental firms and keep abreast of their charges and policies. Often the travel agents have access to a computer with fairly up-to-date rates for the nation's larger firms. For rentals abroad, some travel agencies are able to obtain contracts from foreign firms they use regularly providing for reduced rates to their customers.

Perhaps a fly-drive package that includes airline ticket, some hotel accommodations and a rental car best suits your budget. American, Trans World Airways, USAir and United are among the many airlines offering this arrangement. An array of other packages adds to the bewildering number of choices. The Radisson hotel chain and Budget Rent a Car, for example, are offering a free Lincoln for each night you spend in a Radisson hotel in 16 U.S. cities.

On your own, you can phone as many toll-free central reservation numbers (in the Yellow Pages) as time and temper permit. Or wait until you arrive, and then find out what kind of terms the strictly local firms are offering. But, in either case, before you do so, determine what it is you want. You then start asking questions until you are satisfied you know what you are getting. Among the considerations:

* If you like the convenience of an airport location, then you should expect to pay a little more. The same holds true if the security of a 24-hour emergency road service is important. Or, usually, if you want the newest, cleanest, lowest-mileage car. ("Rent-a-wreck" companies have emerged to cater to a clientele for whom none of these is more important than cheap rates.)

* If thereare only two of you, and all you will be using the car for is to get from hotel to beach, than one of the minis probably is fine. If you are a family of four on a long, mountainous haul with lots of crawling in and out to sightsee, peace of mind requires a vehicle substantially larger, preferably with four doors. Your choices tend to be economy, compact, intermediate, full-size and station wagon, with the price increasing with the size. You probably won't be able to reserve by make and model.

* Do you plan to return the car to where you rented it? If not, the drop-off charge is a big consideration.

* For long-distance driving, you will want to steer clear of limited-mileage rentals if at all possible, says the American Automobile Association. But if you are not going very far anyway, then they could be the best deal if the basic rate is cheaper.

* Are you driving to remote places? Maybe you don't want to gamble on an older car more likely to break down.

* Does your own auto insurance cover a rented car? If so, that should help you in deciding whether you want to skip paying the collision damage waiver.

* Are you entitled to a business or government discount? They often are available for vacation trips. Be sure to ask the reservation or rental clerk if your employer qualifies.

* Does the car come with radio, air-conditioning, automatic transmission? Don't assume that it will if you are going for the cheapest rates. If you are carrying a lot of luggage, you may want a model large enough to have a locked trunk.

* The best rates generally are for weekly rentals for five to seven days.

* Usually you will get a tank full of gas at the outset, and if you return the car full, you aren't charged anything extra. But the policy at Alamo, a small but growing budget firm in some of the busiest rental cities, is to send a car out with $7 worth of gas. Your best bet is to return it gasping for fuel since you will be charged $7 anyway, empty or full.

Getting the right price is one thing; having the car waiting for you when you specified is another. Always ask for a reservation confirmation number. Most larger firms provide them, and they are useful if there is a problem with your reservation. Even with computerized systems, the largest companies sometimes can run short of cars, if only temporarily. The policy usually is to offer a larger-sized car at no extra cost if the size you reserved is unavailable (though you may end up paying more in gasoline costs).

Sometimes, though, a firm simply may have no more cars of any size to be rented. In that situation, company policies vary. They may help you get a car from another firm or, in the case of Alamo, pay the difference between the Alamo cost and that of another firm.

Recently at the Annapolis outlet of a major firm, a visitor from California found no car waiting, no on-site record of his reservation (though he had a confirmation number) and no car at all available for hours to come. In a hurry to reach Richmond, he phoned the central reservation number to complain. Somehow there had been a glitch in communications between the reservation office and the rental outlet. Apologetic, the company offered the Californian free taxi fare to the next closest outlet--a $20, half-hour ride to Baltimore/Washington Airport. He accepted the solution, though it caused him a delay of more than an hour.

Airport locations seem to have the most assured supply of cars, because the turnover is great. You also have the option of quickly checking availability at a number of rental counters. At a hotel location, when a large convention group is checking in or out at the same time, there may be delays.

Before driving away, make a quick check for worn tires; faulty locks, lights, windshield wipers; inoperable windows, heating or air-conditioning system, especially if you are going on a long trip. If you are paying a premium price for a new-model car, question any excessive mileage registered. If you are not satisfied, ask for a substitute. One additional complicating factor in a rental car choice appears to be on its way out: free gifts from tennis balls to TV sets awarded in recent years to frequent customers. Budget halted its giveaway program in February, and Hertz and several others plan to do likewise at the end of June.

Anything that makes renting a car simpler is welcome.