Every few seasons a particular fashion theme spins off from a movie or television show, politics, a tradition, a uniform, even a geographical area where an influential designer lives, or vacations. We've dressed up and played Russian princesses, the Great Gatsby, safari scouts, gypsies, gangsters, street people and endless others.

This year, for spring, the ideas are not so dramatic. In keeping with the more conservative mood of the times, it's the prairie look and nautical influence. And for those who want to look even further back, some clothes that look as if they're right off the set of the popular "Brideshead Revisited" TV series. Nothing very adventuresome, but in a difficult economy, the temptation is to stay with the safe and familiar.

The designer version of any look is, of course, usually far more expensive (as in $100 flounced cotton skirts) than the original idea. There are, however, always the "knockoffs" (copies) that follow.

Some sources of three popular looks and some ways to start emulating -- at a much lower cost -- the designer versions for spring:

The prairie look: Starts with a long, flounced cotton skirt, sometimes worn with a petticoat beneath, and a soft blouse or ruffled edged shirt. Easily traceable to Ralph Lauren's Sante Fe connection (where he has vacationed with his family), variations extend to real chamois and cotton denim, and for summer, even cotton gauze. The look mixes well with another recent Lauren theme, also a spinoff from his attraction to the Southwest: American Indian patterns and colors. (The authentic hammered silver concho belt which Lauren shows often -- available from collectors for $1,000 -- shows up in a silver-metal copy -- $9 and up -- at, among other places, Small Stuff at Tysons Corner.)

The sailor look: Yves Saint Laurent always has been partial to the sailor theme in everything from sailor-cut pants, middy blouses and duffel coat in a crisp mix of navy and white. The sailor look for spring -- in every price range -- is awash in dresses and sportswear with middy collars, broad stripes, and even anchor details. Menswear has its own share of nautical-striped sweaters and white pants. The least expensive sailor suits, of course, are the originals, from surplus stores and secondhand shops.

"Brideshead Revisited": Pastel colors for men were on the drawing boards of some designers -- including Jhane Barnes, Alexander Julian and Alan Flusser -- long before "Brideshead" was aired in this country, but the British-made TV series is bound to give it a boost. Watch for soft tones in three-piece suits, sweaters worn under jackets and front-pleated trousers.

There's wide price range in the "Brideshead" look, from designers' real linen and silk tweed ensembles in the $400 area, to "linen-like" versions for about one-fourth the price.

Women will get their chance to play "Brideshead" this fall. Designers are already drawing dropped waistlines, low-belted shapes and cloche hats. CAPTION: Pictures 1 through 3 and Illustrations 1 through 3, Design inspirations (photos) and their spinoffs (sketches), Ralph Lauren's prairie look in cotton and linen, about $400, a version from The Gap, about $46. Yves Saint Laurent's sailor outfit, about $485, Classic Clothing's middy and trousers, $10 each, gobs' hat from Sunny Surplus, $1.99. Sebastian in "Brideshead Revisited," and Hecht's translation in a linen-look Levi jacket, $70, pale yellow cotton-blend trousers, $26, to wear with pastel shirt, tie and sweater. The teddy bear? $12. Drawings by Martha Vaughan; design by Carol Porter