Among travelers, frequent-flyer mileage credits have become a new form of wealth -- and, says Randy Petersen, who keeps a close eye on airline bonus programs, these credits ought to be spent as carefully as money. The wise traveler is savvy to the strategies of using the earned miles most effectively.

"If you hear somebody at a party tell you they've got '400,000 in American,' they don't mean the stock market." Instead, he says, they probably are boasting that they have earned 400,000 mileage points in American Airlines' AAdvantage Program, good for free tickets or seating upgrades on domestic and international flights.

Petersen is editor of Frequent, a monthly newsletter aimed at keeping subscribers abreast of developments in the frequent-traveler programs of airlines, hotel chains and car rental firms. He has just completed a survey of 47 of the programs, he says, talking to the people who run them. His conclusion: The programs are here to stay for a long, long time. They continue to be effective marketing tools for the travel industry, and consumers love them.

But there are problems with the programs, travelers have grumbled, and some class action lawsuits have been filed. The sorest point seems to be that free seats are not always available when you want to cash in credits for a trip. Airlines typically black out holiday periods, which are popular travel times for families. And the number of free seats on any one flight is limited.

Petersen thinks travelers have expectations that may be too high, perhaps as a result of airline promotion campaigns touting free travel as a benefit of membership in a frequent-flyer program. The fact is, however, that a lot of travelers have earned free miles, and there aren't seats enough to fly all of them at the same time to the hot spots -- Europe in the summer and Hawaii over the Christmas holidays.

John Holland, publisher of Businessflyer, another newsletter for frequent travelers, agrees. He thinks cashing in mileage credits for free seats this fall may be particularly difficult. In the spring, several airlines held promotions offering a free round-trip ticket to passengers who completed a certain number of round trips within a specified time frame. Most of these bonus tickets can be used only from now until mid-December. As a result, the rush is on to take advantage of them.

"There are billions of miles floating around out there that people have collected," says Mary Hunt, editor of Business Traveller International, a magazine aimed at the 15 percent of the business travel market that flies regularly to foreign destinations.

Travelers can argue that airlines should make more seats available for free flights, but no airline wants to fly without a good load of paying passengers -- particularly during the busy season. "Give the airlines a break," says Petersen. "Somehow, they've got to make money."

From his perspective, the airlines may have brought about the problem because of the generosity of their frequent-flyer programs. A while back, they went on a triple-miles binge, giving triple mileage credits for flights -- which delighted passengers who built up a huge bank of bonus miles as a result. And at least for the time being, the promotion is haunting the carriers. Too many people want to cash in their credits at the same time for the same places.

Continental Airlines is among the carriers cited in press accounts this summer as having difficulty meeting the request for free flights to popular destinations. "Continental's frequent-flyer program is probably the most generous in the industry," says Petersen, "too generous. And now it's coming back to bite them in the {rear}."

Continental spokesman David Messing acknowledges that some members of its One Pass frequent-flyer program did not get the flights they wanted this summer, but the overall percentage of disappointed passengers was small. "Nine out of 10 One Pass members got the reward trip they wanted when they wanted it on their first request," he says. The other 10 percent were competing for what he calls "a finite number of seats" to Hawaii and Europe. But half of them "were satisfied with an alternative date or an alternative destination."

The demand "outstrips the supply," he says. Even Continental's paying passengers couldn't always get the flights they wanted. "We ran out of seats to sell in summer."

USAir also could not meet the demand for free seats to London this summer, says spokesman David Shipley. One reason is that USAir has only one flight daily; it is from Charlotte, N.C. Seats to Europe should be easier to get now, however, because the carrier has inaugurated a daily flight from Pittsburgh to Frankfurt.

Despite these glitches in the frequent-flyer programs, the Department of Transportation receives few if any complaints about them from passengers. In July, 682 complaints were filed with the Office of Consumer Affairs, most of them dealing with flight delays and lost luggage, says spokesman Bill Mosley. In a category called "other," there were 21 complaints. These may have included gripes about mileage programs, but the listing is not specific.

Airline officials and industry watchers such as Petersen, Holland and Hunt all agree that travelers wanting to take advantage of frequent-flyer credits must make a seat reservation months in advance, especially if they plan to travel to popular destinations at the height of the tourist season. "If you are going to London, Scotland or Paris next summer," says Holland, "now is the time to lock in your tickets. For any ticket, apply three months in advance to get the flight you want."

In his conversations with airline, hotel and car rental companies, Petersen says he spotted several trends:

The frequent-traveler programs are here to stay. "In 100 percent of the cases, they said the programs will be around in the future, for five years or 10 years." They are popular with consumers and are effective marketing tools.

The "go-go" growth of the programs in the '80s is over, and the airlines should be in a position to more adeptly manage them than they have been in the past. As Petersen notes, the airlines all started from scratch with an initial member, and now they have millions. During the period of rapid expansion, the carriers weren't always sure how to handle the creature they had created.

The airlines will make good use of the database they have collected on their membership. Petersen expects them to develop limited local and regional incentives to fill airplanes. Frequent flyers can also expect more and better service, since they are the passengers the carriers want to keep loyal.

Award levels probably will not change much. In 1989, most of the major airlines increased the amount of mileage credits needed to earn free flights and seating upgrades, and they put a time limit on when the credits could be used. The move caused an uproar. Petersen concludes the industry doesn't want to anger its best customers again.

Airlines will introduce new and unusual programs to attract frequent flyers. Petersen cites Eastern's current Business Break promotion aimed at business travelers using full-fare tickets. Passengers who fly seven round trips from Sept. 2 to March 1 on specified days of the week earn two free tickets to Hawaii, Cancun or the Virgin Islands.

However, the winners will not be flying on regularly scheduled flights, where they might be competing with fare-paying passengers for seats. Instead, Eastern is arranging a series of special charters to these beach destinations between Dec. 14 and April 14. Only the winners in this promotion and a companion will be aboard.

Participants in the American Airlines AAdvantage program can earn miles each time they buy bonds, stocks and certificates of deposit. A brochure explains that an AAdvantage member who opens a $50,000 six-month certificate of deposit earns 500 mileage credits a month for six months -- in addition to interest on the investment.

Travelers who belong to more than one program are well aware that each is different, and each has an odd quirk or two that can trip you up if you aren't careful. Petersen, Holland and Hunt recommend these strategies as a way of making the most of your mileage credits:

Book early: It may be inconvenient, but the facts are that free seats and upgrades are limited and they are snapped up quickly to popular destinations. If you can't get the flight you want but have lots of miles to your credit, Holland suggests writing a letter to officials of the airline. For a good customer -- one who flies many thousands of miles a year -- they may be quite willing to relax their capacity controls and make a seat available.

Be aware that mileage credits can expire: When they cut back their award levels last year, several carriers -- including United, American and Pan Am -- set time limits on when the mileage credits could be used.

For example, any mileage credits earned on Pan Am in 1991 must be used by 1993. Pan Am says this permits passengers to use their credits "for up to three years." The catch, of course, is that you have to earn the credits on Jan. 1, 1991, to get the full three years. If you happen to travel Dec. 31, 1991, then your credits expire in two years.

Know the difference between old miles and new miles: Depending on the program, mileage credits earned through 1988 are more valuable than credits currently earned. This is true because the old mileage can be used to obtain the more generous rewards offered prior to 1989. For example, on United you are entitled to a first-class upgrade on domestic flights for only 10,000 old miles; with new miles, the requirement is 20,000 credits.

Also, old miles are easier to spend; United sets no capacity controls for free flights and upgrades. However, controls are applied for new miles. Sometimes you can get what you want with old miles when the same thing isn't available with new miles, as a Washington traveler learned this summer. He wanted a first-class upgrade on the long, overnight haul from Alaska to Washington. Nothing was available if he planned to use 20,000 new miles, but the clerk managed to accommodate him when he opted to spend 10,000 old miles.

Not only are old and new miles different, but so is the way airlines apply them. United's old miles expire in 1994; there is no expiration date for American's old miles.

Don't overlook opportunities to earn mileage credits: Petersen estimates that frequent flyers can earn 40 percent of their mileage credits from specified airline partners such as hotels and car rental agencies. You get bonus points for staying in specific hotel chains and renting from specific car agencies. But many travelers don't bother to keep track of which partners an airline has signed up.

The two newsletters, Frequent and Businessflyer, are designed to keep business travelers abreast of changes in frequent-traveler programs. Each is published monthly. Business Traveller International, a monthly magazine, provides coverage on a variety of topics of interest to business travelers abroad.

For information:

Frequent: 4715-C, Town Center Dr., Colorado Springs, Colo. 80916, (719) 597-8889. Subscriptions are $33 a year. Publisher Randy Petersen also offers a mileage credit tracking service he calls the Frequent Flyer Club. For a fee, he will send you a monthly report summarizing your mileage account for all the programs in which you participate. He will obtain vouchers for you and warn of approaching expiration dates. Depending on the level of service desired, the fee is $50, $100 or $150 a year. For information on the club: 1-800-333-5937.

The Businessflyer: P.O. Box 276, Newton Centre, Mass. 02159, 1-800-359-3774 and (203) 782-2155. The subscription rate is $50 a year, which includes a $50 U.S. Savings Bond.

Business Traveller International: 41 East 42d St., Suite 1512, New York, N.Y. 10017, (212) 697-1700. A subscription is $36 a year.

Travelers who have complaints about airlines, including any difficulties involving frequent-flyer programs, can telephone the Consumer Affairs Office of the Department of Transportation in Washington, 366-2220.