I would have felt pretty ridiculous, waltzing up to the room clerk at San Francisco's St. Francis hotel this winter and saying, "Sourdough." On the other hand, I would have been semi-permanently vexed if my finickiness had kept me quiet.

"Sourdough" was one of the season's magic words. Saying it would have got me $20 off the bill for any stay of two nights or more.

How do you find out about things, like this? With some difficulty. That's why anyone who wants to cut travel costs this summer had better start work now. Along with buying ahead, joining a travel discount club and learning to time your travel for the utmost price advantage, skillful shopping is the answer to saving.

Enter catalogs and directories. Basically, there are two kinds. Many are sent free (or for very little money) to anyone who requests them; others are publications for profit that may or may not be worth the price charged. Some of the latter kind, though, are available at libraries, so that's as good a place as any to start a search. What might be especially useful for the cost-conscious traveler this summer? Well, consider these;

Rent a Tent. You say you're too squeezed to pay more than $5 a night for vacation lodging? You say you'd still like truly scenic surroundings, but snuggled up close to places where there's "something to do"? Well, Oklahoma may have just the answer.

If you write the Oklahoma Tourism and Recreation Department, 500 Will Rogers Building, Oklahoma City, Okla. 73105, they'll send you news of their Rent a Tent program. As of June 1, they'll have a batch of tents picked in 10 state parks. Each will have two cots with mattresses, an outdoor grill and battery-operated lantern, and you could bring a bedroll to slip in an extra person. The bottom line is that you can reserve a camp for up to 14 nights, at $10 per night per tent.

Switzerland on the Cheap. That's cheap as in "comparatively speaking," but roughly $35 a day will get a village apartment for two in the lake-dotted alps, and the per-person price goes down for larger units with more people sharing. The setup involves three-week stays, but you can arrange variations, all including "support services" in the form of newsletters, escort from Zurich airport and orientation sessions.

The operators of the program, Idyll Lt., P.O. Box 405, Media, Pa. 19063, are eager to promote people-to-people vacations; in fact, the business was prompted by inquiries stemming from a book, "Shoestring Sabbatical," written by one of the company's founders, a teacher. "Informational brochures are free.

Other Alternatives to Hotels. SAS airlines would like to tell you how to book a farm vacation or find a low-priced holiday cottage in Norway, Denmark, Sweden or Finland; you can pick up a free copy of "Low Cost Vacationing in Scandinavia" at any of their ticket offices, or write to SAS, Box AU, Suite 14654, 630 Fifth Ave., New York, N.Y. 10020.

The Irish Tourist Board, 590 Fifth Ave., New York, N.Y. 10036, would be equally glad to instruct you, via a directory, on renting a nicely equipped seven-room cottage in any of 14 villages from Galway to Tipperary. This summer the high-season rate will be about $300 a week.

Rental cottages and apartments all over the English countryside are explained in brochures put out by a large number of companies, among them: Country House Holidays, Borehamgate House, Sudbury, Suffolk C010 6 ED; Home from Home in England, 30a High St., Haslemere, Surrey GU27 2HJ; Lakes Holidays, Wroxham, Norwich NR12 8DH; Swiss Chalet-InterHome, 10 Sheen Road, Richmond, Surrey TW9 1AE. Price range is roughly $200-$250 a week. (Incidentally, when querying any source abroad, it's best to enclose return postage in the form of an International Reply Coupon, available at post offices.)

How much for a small inn oozing charm and comfort? Well, the Spanish government's much-acclaimed "paradores" (inns) this summer range in price from $14.50 a night single to $63.50 double, but the average is $22-$25 single, $35-$40 double, tax and service included. In some places, prices are slightly lower before June 15 or June 30 and after Sept. 15. Ten-day stays are usually the maximum allowed. This year you or your travel agent can get more information and make reservations here through Marketing Ahead Inc., 515 Madison Ave., New York, N.Y. 10022, (212) 759-5170.

These are just the tip of the iceberg. Many tourist offices and travel agents can, if they will, steer you to other opportunities.

Closer-to-Home Options. Camp with Indians, right on a reservation? Yes, it's possible -- and by mid-April, travel agents should know where. That's when they'll receive the American Indian Travel Commission's to-the-trade directory of hotels, motels and camp-grounds, about 85 percent of them on Indian lands. (Individuals can get a directory then, too, by writing to the commission at Suite 550, First Bank Building, 10403 West Colfax Ave., Denver, Colo. 80215, but the price is not yet set.)

Cheap-sleep tips are in "Where to Stay USA from $2 to $10" (Frommer-Pasmantier). The 1980 edition will be out the end of April, but you can order it now from most bookstores-at $4.95 a copy.

Of course, if you've an old copy (or can find one at the library), you'll be almost as well informed, except for exact prices. Since those don't seem to be too exact anywhere anymore, never mind; use the addresses given for budget motel chains, write for their free directories and you may even come out ahead, since they sometimes contain news of special offers. (You'll also learn that Motel 6, one of the largest and usually the cheapest of the nationwide budget groups, is one of the few with naitonwide rates. At present, they're $10.95 for one person, $14.95 for two and not expected to rise this summer.)

Old but Not Out. Elderhostel this year offers its sixth annual summer schedule of week-long residential academic programs for the over-60s. They'll be "on campus" at some 300 colleges and universities in this country and Canada and $130 is the standard charge for room, board and courses in things like "Activities" (walking, swimming, yoga, meditation, aerobic exercises), "Coping with Conflict and Change," "Colorado's Railroads" (including visits to historic areas), "The Genius of Puccini," "Modern China) and much, much more. A catalog with all details is free from Elderhostel, 100 Boylston St., Suite 200, Boston, Mass. 02116 -- and yes, early reservations are a good idea.

All? By no means. Stay tuned for more next Sunday.