When you have made the big decisions about which posters to acquire, you will soon be thinking about safe storage or framing.

Most museums and dealers store posters laid out flat in shallow drawers. You can buy blue-print sized files at office and art supply stores, if you are willing to make the investment and have the space.

Some possibilities for home storage are: acid-free boxes of the correct size, which can be stored under a bed; acid-free paper folders, one for each poster, which should be laid flat; encapsulation in two sheets of Mylar Type D, taped around the edges (but not touching the poster) with strips of doublesided Scotch tape #415; or a Mylar Type D/acid-free paper combination encapsulation. Mylar has the advantage of being a clear polyester, nonsticking film, keeping the poster visible. All of the above materials are non-injurious to paper. No works of art should come in contact with acid materials; acid causes discoloration and deterioration within a few months or a few years depending on climatic conditions. Conservation Resource, 1111 N. Royal St., Alexandria (telephone 549-6610), can supply any of the storage units described above.

The same care should be taken with framing. If the poster is in good condition, conservationists recommend that it not be mounted on a rigid support. It's better for the paper to be allowed to expand and contract naturally. The poster can be held with hinges (the best are made from mulberry paper and paste) on an acid-free ragboard.

If the poster is damaged, it should be lined with Japanese tissue using wheat or rice paste as an adhesive. And if the poster is extra-large or more seriously damaged and a rigid support is desirable, it should first be lined and then mounted, using an archival adhesive, on a metal or cardboard grid panel which has been faced with Mylar, ragboard or thin aluminum sheeting.

Because of the large size of most posters, framers usually prefer to use plexiglass, much lighter in weight than regular picture glass. But plexi should be kept from coming in contact with the poster either by using a two-track frame or spacers with a regular frame. Best of all for the purpose is ultraviolet plexiglass (expensive), which in addition to being light in weight, filters out a good percentage of harmful light rays. Light, and especially sunlight, is injurious to paper.

Restoration of damage (tears, holes, surface dirt, stains, etc.) should be undertaken only by a professional paper conservationist.

You can get expert advice on poster conservation problems by calling the Restoration Office of the Library of Congress, 287-5635; or Fern Bleckner, Paper Conservation Department, the Corcoran Gallery of Art, 638-3211.

The National Collection of Fine Arts, 8th and G Streets NW, offers a free conservation clinic (not just posters) every Thursday from 10 a.m. to noon. The staff gives professional opinions on the condition and history of your work of art. (They do no private restoration work nor do they make appraisals.) They urge you to call before coming (357-2685) since it often possible to solve a problem by phone.