WHAT POISTERS should you buy to day? No one can say this poster will be worth a million dollars 10 years from now or that poster will give you joy forever. Even the experts can't say what the trends will be.

Dealer George Theofiles does think that the boom may have pushed Art Nouveau to unsustainable highs; and Library of Congress curator Elena Millie finds Polish posters among the most interesting being produced today.

But all agree that you should first educate yourself by looking and reading and then buy what pleases you. You should let no one influence you about style or subject matter. But you should buy the best you can possibly afford; original posters in good condition.

A copy will never be worth much. And how can you tell if a poster is an origianal (meaning an example from the first printing however produced: stone lithograph, offset lithograph, woodcut, offset photo, silk screen, etc.)? With so many methods of production, there can be no universal rule. Until your eye becomes sharp, you will have to rely on a reputable dealer.

A good dealer can also be of help with the important consideration of condition. George Theofiles writes: "Truly pristine mint specimens are in some cases worth as much as double those of the same poster in 'average' condition." He suggests looking for colors "out of register," tape repairs, stains and discolorations, excessive rips or tears, folds, crumpling at edges and quality of paper -- though the importance of the faults can vary.

You can educate yourself by visiting museums, auctions and dealers and by reading.

All art galleries and museums in Washington and throughout the country sell posters, sometimes made to promote their own shows (King Tut at the National Gallery of Art had several) as well as posters from other museums.

The Great Midatlantic Antiques, Advertising and Memorabilia Show & Sale will take place on Sept. 6 and 7 at the Gaithersburg, Md., fair grounds. Organized by Hubert Bellman, the sale (in its 16th year) includes "anything relating to advertising and is very heavy on posters." Visitors come from as far away as the West Coast.Robert Gotsch was commissioned in 1972 to develop a poster for the Show & Sale and it costs $3. c

The Hirshhorn Museum is exhibiting "Art Deco Posters from the Collection of the Library of Congress" until Oct. 5.

You can learn something about posters by making your own design at the National Archives Poster Workshop for amateurs to be held on Sept. 30. tThe cost is $5. More information can be had by calling 633-6983.

An exhibition called "Collective Visions, National Poster Art" is at Local 1734 Art Gallery, 1734 Connecticut Ave. NW until Sept. 7.

The Martin Luther King branch of the Public Library lends posters.

The Library of Congress has one of the most comprehansive poster collections in the world. To see examples of the subject matter, or period of history, or artist of most interest to you, call the Curator of the Poster Collection, Elena Millie, at 287-5844.

Among the dealers in Washington handling original posters are:

Summer Squash Gallery, 3212 N Street NW. This is the only gallery in Washington dealing exclusively in collector-quality original posters. Chip Nourse is in charge of the large selection; he knows a great deal about the history of posters and can help with the problems of orginality and condition. The prices on the average are from $15 to $100, but there are some extra-fine examples available costing in the $1,000s.

Inglett-Watson, 925 1/2 F Street NW. Dan Inglett and Gene Watson: deal in fine quality Art Deco furniture and art, including a very nice stock of original posters from that period. They cost from $15 to $1,000.

Time & Again, 2900 M Street NW. Steven Bauman handles original Art Deco posters, mainly European. He has an excellent understanding of the problems of condition and conservation, and can give helpful advice.

Foliograph Gallery, Tysons Corner Center, McLean, and 1821 K Street NW. This gallery handles original contemporary posters. Some may be the collectors' items of the future. Prices range from $8 to $85 (most between $10 and $30). In addition to offering their stock of 3,000 titles, the owners, Jack and Stephanie Herman and Joseph Wetzel, are glad to try to locate unusual posters on request.

And not too far from Washington:

George Theofiles' headquarters in New Freedom, Pa., (about 40 miles north of Baltimore), is bulging with posters and advertising ephemera. Called Miscellaneous Man it offers probably the largest mail-order selection of original posters in the world. An excellent catalogue, including prices (very often under-market) and professional descriptions, is issued once or twice a year. Most of the listed posters are quickly sold. He welcomes queries and, if he doesn't have exactly what you're looking for, he probably knows where to find it. Miscellaneous Man, Box 1776, New Freedom, Pa., 17349. Call (717) 235-4766 (10 a.m.-6 p.m.).

The following auctions will be held in the fall in New York City:

Phillips next poster auction on Oct. 24 will offer approximately 200 lots, all by Jules Cheret. The Nov. 15 poster auction will offer about 500 lots by a variety of artists. For further information or to order a catalogue call Nancy McClelland, (212) 570-4890, or write to her at Phillips, 867 Madison Ave., New York, N.Y. 10021. (The two back catalogues for last November's and May's sales are also available. They include prices realized, condition, etc.)

Sotheby Parke Bernet will include about 150 Belle Epoque posters in its Nov. 1 auction. Estimates are from $800 to $18,000. Of perhaps even more interest in the auction will be a small selection of unusual posters by such artists as Antonio Gaudi (Barcelona architect), Magritte, and Jean Dupays (designer of the Normandie Liner). For a catalogue write to Helen Wellner, Catalogue Subscription Dept., Sotheby Parke Bernet, 980 Madison Ave., New York, N.Y. 10021.

Christie's will include about 215 posters from the period 1895-1905 in its Sept. 24 and 25 auction. All are from one consignor, all are ads for manazines and books, and by such artists as William Glackens, Maxfield Parrish, John Sloan, Edward Penfield and Louis Rhead. Estimates are from $200-$400. On Nov. 15 there will be an Art Deco auction with some posters by Cassandre. Write to the Catalogue Subscription Dept., Christie's, 219 E. 67th St., New York, N.Y. 10021.

Some books to look for are: "A Concise History of Posters" by John Barnicoat, "The Poster" by Bevis Hillier, "Poster in History" by Max Gallo, "American Posters of World War I" by George Theofiles, and an excellent catalogue called "Images of an Era," published in 1975 for the Corcoran Gallery's exhibition of that year, available at the National Collection of Fine Arts shop.