(MONEY Magazine) – Wherever you are flying this summer, whether down to Rio or up to old Cape Cod, you had better be prepared. It's a jungle out there. To book a flight intelligently, you have to hack through an undergrowth of air fares, rules and restrictions. And you are more likely than ever to endure delays and cancellations. While you can't do much about being grounded, you can learn how to snare an affordable fare.

Airline deregulation may have caused some ills, but it has also produced terrific bargains. With the lowest fares, such as the MaxSaver from Continental, you can save 60% to 80% off full-fare coach prices. Flying from Boston to San Francisco in late May, for example, you would have paid $278 round trip in coach instead of $680. The snag is that cut-rate seats are often scarce, always conditional -- requiring advance purchase, for example -- and beset by punishing cancellation penalties. To ferret out a low fare that you can live with, you need to understand the computerized method to the madness. From the moment an airline begins to accept reservations on a flight until the plane takes off, computers continuously monitor and adjust the number of seats being sold at various price levels, adding or subtracting discounted seats in response to supply and demand. So in spite of advertisements headlining bargains, you may find low fares elusive, particularly on heavily traveled routes or at popular flight times. Says Daniel Smith, director of consumer and industry affairs at the Airline Passengers Association, a business travelers' group: ''You almost have to call the airlines first before you put in for a vacation.'' And you probably have to make those calls yourself, because not many travel agents will search out a cheap fare that gives them little or no commission. The best time to get through to the airlines is after business hours. Before you call, however, ask yourself the following four questions: -- How flexible are you about when you fly? The looser your plans are, the better. By timing your flight for days and hours when business travel is lightest, you will get your best shot at cheap seats. Many airlines chop $10 to $30 off prices on most routes during off-peak days, usually in the middle of the week. The table below, based on airline records, shows the times at which business people tend to fly -- all day on Monday and Friday, for example. To unearth the rock-bottom fare, tell the reservation agent who answers your phone call that you haven't decided when you want to fly but that you want the cheapest available fare. To get the deepest discounts, you might have to adjust the length of your visit as well. The condition is often that you stay over a Saturday night. You can also sometimes cut your costs by not insisting on a nonstop flight. For example, TWA was offering a $1,040 round-trip coach fare in May on nonstop flights between New York and Los Angeles but charged only $560 with a layover in St. Louis. -- How far in advance can you book the flight? In general, the longer before flight time you buy your ticket, the less it costs. For example, by purchasing tickets on Pan Am's flight from Chicago to Paris 21 days in advance, you would have paid $777 round trip in late May instead of $1,424. -- How late can you wait to make your plans? If a flight is undersold, an airline may put more discount seats on sale right up until departure. So it pays to keep phoning. -- Can you live with nonrefundable tickets? Most discount seats carry penalties ranging from 10% of the fare to total forfeiture when you have to cancel or reschedule your flight. Airlines usually do make exceptions in the event of illness or a death in the family if you can provide proof, such as a letter from your doctor. Refund policies differ, however, so be sure to ask what they are. Once you know your limitations, you can figure out which of the airlines' terms you can live with. You should also be aware of some strategies that could lead to savings. For example, if you are willing to pay a little extra as a hedge against changes in your travel plans, you can still save money by buying more than one round-trip nonrefundable ticket and using only the portions that fit your schedule. Example: You could buy four of United's nonrefundable $258 round-trip fares between New York and Los Angeles, each for a different flight, and still pay a bit less than the $1,040 full coach fare. Roger Scott, associate editor of Best Fares (P.O. Box 171212, Arlington, Texas 76003; $78 for 12 issues), describes a strategy that involves splitting a round-trip ticket between two airlines. Say you find an irresistible fare on one airline, but it doesn't have seats available when you want to return. ''Reservation agents have other airlines' schedules and fares on their computers,'' says Scott. ''If you ask, you can often combine airlines, and the discounted fare is still valid.'' He also suggests probing for a bulk fare, a % seat that the airlines usually sell at a $30 to $50 discount as part of a vacation package. Have the agent connect you with the tour desk and ask whether you can buy any unsold seats on a package flight. On international flights, many travel agents traffic in bulk-fare tickets bought from wholesalers and retailed at 15% to 50% off regular fares. Don't expect logic always to rule the fare structure. Paradoxically, first- class seats are sometimes cheaper than coach. On the Boston-San Francisco Continental flight mentioned above, you could pay $458 for a first-class ticket purchased seven days in advance and still save $222 off the unrestricted coach fare. After you have found a price that suits you, don't let it get away. The average life span of air fares these days is about two weeks. But your price is guaranteed once the airline prints out and sends you the ticket. What if the fare you paid goes down before your departure? You will be entitled to the lower one. But it's up to you to stay abreast of the latest fares and insist on any refund you are entitled to. You should call the airline periodically to find out about fare changes, or at the very least inquire when you check in for your flight.

CHART: TEXT NOT AVAILABLE CREDIT: PETER KUPER CAPTION: WHEN THE LOW FARES FLY Discount seats are most abundant when business travelers don't take to the skies. Your chances of flying on the cheap are best during the light-demand periods in green. DESCRIPTION: Light, medium and heavy business use of airlines for four time periods on each day of the week.