If you sincerely wish to save money on vacation costs this summer, do you really have to spend your spring becoming everyone's pen pal? No, but it certainly does help.

To begin with, no one can determine what races your blood as well as you can. Furthermore, you can't really "cut" costs until you know what they are. In the end, that means holing up at the library, reading guidebooks and travel articles in order to get together the names and addresses of all the fine folk who want to send you free information: tourist offices, airlines, hotel chains, tour companies and car rental firms among them.

Is it worth it? Well, the Howard Johnson free directory, for instance, lets you in on news of special promotional rates. It places a "P" beside the name of each inn that has any kind of discount. The Days Inn free directory tells about its discount club for travelers over 55. National Rental Cars lets out word that its European division, Europcar, this summer will guarantee its rates in dollars: $145 a week unlimited mileage for its smallest cars in any country, $195 for Volkswagen Rabbits or similar.

Conceivably you'd also learn that roughly half the campgrounds in the National Parks system are free; that Econo-Travel, at about 40 percent of its budget motel properties, offers the seventh night free to anyone who spends six nights; that on New York-to-Nassau cruises aboard the Rotterdam two full-fare payers may bring along one or two more people to share their cabin for free through July 26.

Armchair shopping steers you in other useful directions, too. Besides those avenues mentioned last Sunday, these have welcome news for the money-minded:

Guidelines for the Young and Young-at-Heart. You don't have to be a student to profit from reading the 48-page 1980 Student Travel Catalog. It's got information on charter programs open to people of all ages this summer (round trip to Amsterdam $593, Paris $449, Tel Aviv $749-$999; one-way fares also are offered) and serves as a good aid generally by quoting prices on things like European car rentals and intra-European air fares.

It further directs readers to good guidebooks, tells about work programs abroad, lists some operators of expeditions that might appeal to adventure-some travelers aiming at Europe, Africa and Asia. Also, it's hot off the press and costs only 50 cents from the nonprofit Council on International Educational Exchange (Student Travel Services), 205 E. 42nd St., New York, N.Y. 10017, so you can't really lose.

Swap and Save: In 1980, home exchanges look like an idea whose time might have come. What's also worth noting is that most exchanges involve wheels as well; you get around in their car while they get around in yours.

You can advertise for a swap on your own, of course. However, there are a number of outfits that, for a fee, provide "introductions." Among the most active: the Vacation Exchange Club, 350 Broadway, New York, N.Y. 10013, and Inquiline, 35 Adam St., Bedford Hills, N.Y. 10507.

Vacation Exchange's latest directory just came off the press and has names of about 6,000 would-be swappers, two-thirds in the United States, Canada and Mexico, the rest overseas, for a total of 40 countries. Directories are published in February and April, and $12 buys both editions. A listing will cost you $3, plus an additional $5 if you want them to print a photo of your home or family. Inquiline will have about 1,000 listings in its latest directory (about 80 percent of them Americans, the others mainly in Britain, France and Germany). They charged $30 a year to list you, $20 more if you also want to receive a year's worth of directories, supplements and newsletters. (You can buy the publications package without being listed for $30.)

It's already too late for you to be listed in most of the summer directories, but in any case, before you sign up, it's advisable to ask first for a free explanatory brochure.

For Travelers Rather Than Vacationers: Norman Ford, a man who truly "pioneered" low-cost travel with his books for Harian Publications, also founded a group called the Globetrotters Club, which today directly and indirectly keeps wanderlust alive with a newsletter. "The Globe" offers a monthly potpourri of news few people can use (personal accounts that invariably get into things like where to find a cheap place to stay in the Arctic Circle, what bridge was just washed out in Kenya). Yet in sum it reveals much.

A year's membership, $12, gets you not only air-mailed newsletters but a directory listing other members around the world who'll share what they know about given places. Write: Globetrotters Club, BCM-Roving London WC1V 6XX, England.

Anyone for Adventure Trips? If you don't know where to find them, the American Adventurers Association (Suite 444, 301 Northeast Ravenna Blvd., Seattle, Wash. 98115) does. Membership is $25 a year, for which you get a monthly magazine and a travel guide -- plus answers to any specific questions you have about the more than 3,000 commercially organized adventure tours now operative. Or you can get the guide alone for $9.95 at bookstores ($1 more by mail); the 608-page paperback, "1980 Worldwide Aventure Travelguide," is crammed with trip details, prices and operators.

Learning Vacations: Everything you've always wanted to know about summer school, family or adult variety, is not out in updated form, but you can get a copy of last year's "Learning Vacations" by sending $6.95 to 2 Hamill Rd., Suite 327, Baltimore, Md. 21210. Many of the 400-plus programs described are offered annually. On the same principle, you can also look up the April, 1977, Better Homes and Gardens rundown on "Arts and Crafts Family Vacations."

Finally, when you have plenty of information under your belt, you're in 100 percent better position to have a productive talk with a travel agent or to plunge ahead on your own. Obviously, saving money costs time, but since it also pays off in helping you get what you want, it's a good idea to give till it hurts.