People once feared it was crass to take restaurant leftovers home, that to ask for the meat and bones to be packed in a bag might mark them as (a) cheap, (b) poor, (c) unsophisticated, or all of the above. Even the tern "doggy bag" has eloquent undertones of embarrassment: "It's not really for me, Mr. Headwaiter, it's for my dog."

For whatever reason, in the past few years restaurant customers have loosened up about asking for the leftovers, and the eyebrows of waiters at even the posh places no longer arch in silent contempt at packing the uneaten filet. In fact, some restaurants make a great show of doing up the leftovers -- your doggy bag may take the form of a faniful, handshaped aluminum swan, ot it may be accompanied by a fresh flower.

Granted that it's okay to speak up about wanting the leftovers, why take the trouble? "Who needs a few chicken wings, or a half-portion of moo shi pork?" you ask. "Besides, we're trying to cut down on snacks." Those questions miss the point of doggy-bag cuisine. The idea is not simply to eat the stuff right out of the carton, but to use it as the nucleus of a whole meal the next day. Are you impressed with the intricate spicing of a Szechuan dish? Are you discouraged at the thought of trying to duplicate that special taste in your own kitchen? Doggy-bag cuisine allows you to recapture the essential flavors, to build a new dish -- similar to but not identical to the original -- on the foundation already laid down by the restaurant chef. Look at that little carton as the potent concentrate that you'll skillfully expand into a new dish. What To Take Home

The advanced doggy-bag devotee starts thinking about the possibilities for next week's meal early in the game, while ordering in the restaurant. He or she knows from experience that there are more possibilites in moo shi pork or Szechuan beef than in the more delicate, thick-sauced Chinese dishes like shrimp in Lobster sauce. Pungent, highly seasoned soups can either be expanded at home with more broth and seasonings or concentrated into sauces, while subtle, creamy bisques tend to be more limited. If it becomes appartent during the meal that there will be food remaining, the connoisseur of leftovers will eat selectively, finishing, for example, the chicken breast and taking home the more flavorful dark meat and bones. Trying to cut down on red meat? Finish the baked potato and take home some steak. You can work wonders with it the next day.

The ultimate doggy-bag treasure is a duck carcass. Pure gold. Which means having your Peking duck in a restaurant that features table-side carving. (Or, even better, Szechuan crispy duck -- its bolder flavors will expand more dramatically.) Then it's perfectly natural to ask the server to wrap the remains for you.Remember, the carcass you leave on the carving table will be the restaurant's soup tomorrow. Deceptive Labeling

Once you've got the stuff home, beware of children old enough, curious enough or hungry enough to consume your catch before you can work on it. Those carefully hoarded cartons in the refigerator, waiting to be transformed into an innovative dinner for two, can disappear in minutes if they're discovered by a ravenous teen-ager. Guard your doggy bags, and let the kids snack on Resse Cups and Mallomars, like their friends. The best way to insure security, short of buying a second refigerator with a lock, is to intentionaly mislabel your leftover treasures. In our experience, no one under the age of 30 will bother with a container labeled "mung bean sprouts" or "boiled rice." As long as you don't start believing the labels yourself the system is foolproof. The Inspiration Factor

Recipes for doggy-bag cuisine should be taken as reough sketches, to be finished to your taste. To follow the recipes blindly is like painting by number, or missing the rare bird because you were the field guide. Experimenting with those little packages in the refigerator should be play -- finger-painting, rather then number painting. But inspiration needs tools. A few broadly useful items to keep on hand are soy sauce, sesame seeds, fresh giner, thin slices of deli ham and a good quality canned chicken broth. And lots of fresh garlic. In making dishes we seldom measure anything. In the interest of communication, we've worked up some measurements, but take them for what they are: general guidelines. (Note that we've ommited salt; you'll have to let the saltiness of your leftovers be your cue.) Recipes aside, the essential step in the high art of doggy-bag cooking is to open your trove of packages and let your mind, fingers and imagination wander: "What else can I throw in . . . ?" PBLT

Proportions are up to you. The idea is to make your usual bacon, lettuce and tomato sandwich, and them remove it forever from the luncheonette class by adding pate to the filling and moistening with an unusual sauce. Leftover pate Bacon Lettuce Tomato Toasted bread Sauce: About 2 teaspoons green peppercorns About 2 tablespoons mayonnaise About 1/2 teaspoon dijon mustard

Assemble pate, bacon, lettuce and tomato on toasted bread.

To prepare sauce, cruch the green peppercorns (use less if you don't like hot sauces). Add the mayonnaise and mustard. That's it. You can spread the sauce on the sandwich, before serving, or allow people to spread it themselves. (If you choose the latter, make double the sauce. It goes fast.) SESAME STEAK STRIPS

Here's a quick and easy way to turn leftover steak into a fresh, new appetizer. Leftover steak, trimmed of all fat and gristle, and sliced as thin as possible Scallions, finely chopped Soy sauce Chinese hot oil (optional) Sesame seeds (about 2 teaspoons per person)

Arrange a fan of steak slices on individual plates. Sprinkle with scallion. Serve with side dishes of sesae, soy and hot oil. Each dinner mixes soy sauce and hot oil to taste on one side of the plate, and heaps a small mound of sesame seeds on the other. The steak strips are dipped first in the soy-oil mixture, then in the seeds, and then -- scooping up green onion bits on the way -- into the mouth. (Fingers preferred.) Incidentally, the only good sesame is a fresh sesame; discard your old stock: GREEN BEEF STRIPS

Maybe, not hving included beef in your investment portfolio lately, you'd like to serve the leftover steak as a main course. Here's one of dozens of ways to do it. Four ounces of steak is enough for two, but more's better, of course. Leftover steak, trimed of fat and gristle, and sliced into thin strips 1/4 cup water or dry white wine 3 or 4 large mushrooms, sliced (mnce the stems) 2 or 3 stalks fresh broccoli, sliced into thin strips about 2 inches long (stems and all) 1/2 teaspoon fresh ginger Minced garlic to taste 2 tablespoons soy sauce

Reserve a small piece of fat, but throw the rest, together with the gristle, into heated pot. Brown for a few minutes, letting the dark, crusty parts of the gristle stick to the pot. Add the water or wine and simmer gently, uncovered, while you're cutting up the vegetables. When the vegetables are prepared, and your stock is cooked down to half of its original self (skim unwanted fat), get ready fo the cookdown.

Timming counts here. Throw the fat you've saved into a fairly hot, heavy pot or better, a work, and twirl the fat on the end of a fork until you've coated the pot. Throw in the mushroom stems and let them brown, crushing with the wooden spoon as you do. Now toss in the broccoli and stir-fry for a minute or two, depending on how crisp you like it (we prefer it crisp and bright). Put in the sliced mushrooms, the ginger, and the garlic, stirring all the while. And then the steak strips. If you want to keep the steak rare, allow it to just heat through. Finally, add the soy sauce and strained stock. Stir just long enough to allow the vegetables and steak strips to become well coated. Serve over rice. PASTA E POLLO ALLA SACCO CANE

We think pasta addicts will find happiness in this chicken spaghetti sauce, one of our favorites. The base: leftovers from that half roasted chicken you couldn't finish. A leg, a thigh and a wing will do nicely. 1 tablespoon butter 1 medium onion, sliced fine Leg, thigh and wing of leftover chicken 1 cup water, more or less 1 or 2 slices smoked ham, diced 1 clove garlic, minced 1 tablespoon chopped fresh parsley 1/4 cup half-and-half 2 or 3 mushrooms, sliced 1/2 cup leftover peas (optional) 1 teaspoon parmesan or grated swiss 2 egg yolks, beaten Pasta for serving

Heat a heavy soup pot. Toss in about a teaspoon of the butter and, when it's begun to froth, add the onion and brown, stirring as you do. Add the chicken, just as it comes from the doggy bag. If there are slices of lemon sticking to the skin, or flakes of oregano or rosemary, marvelous. Throw them in, too. Add water to cover, and simmer, very gently, with lid off. (Be careful not to let it boil, which will destroy the texture of the meat and suffocate the herbs and spices in which the chicken was originally cooked.)

After the broth has begun to take on some character -- it will darken to a deep yellow and become slightly opaque as it cooks down -- remove from the heat. Put the chicken on a plate to cool. Strain the broth into a bowl, then put it back into the soup pot. (This removes the flotsam of gristle bits and onion slices. If you like flotsam, pick out the best parts and throw them back into the broth.) When the chicken has cooled, remove the skin and bones, and put the meat aside. You probably won't get more than 1/3 of a cup of meat, but that's enough.

In a heavy saucepan, brown the pieces of ham, the garlic, and the parsley -- if the ham has no fat at all, throw in a little more of the butter -- and put that aside, too. Now you're ready to put it all together. The secret to making this dish is to let the sauce cook down enough so that it thickens and coats the pasta, without boiling any of the goodness out. Reduce the broth to maybe 1/4 cup, and add the chicken and ham. Now remove the soup pot from the heat and add the half-and-half, mushrooms, peas, cheese and any remaining butter. Stir until the cheese melts and blends in, then add the beaten egg yolks. Return to the heat and let the sauce heat through. (If you boil now, you'll get your comeuppance: it will curdle.) Pour over hot, chewy pasta -- and savor. FUDGE CAKE PARFAIT (4 servings)

Here's a way to make regal use of leftover cake. There are only a handful of rules here. Stale is fine, but artificial cake, the kind that staggers under a load of preservatives and substitutes, is out. And whipped cream cake is out. If it's good whipped cream, it should be eaten as is, and at once. If it's ersatz whipped cream, it may soften your skin but it won't translate into a good dessert. 1 large piece leftover fudge cake Vanilla ice cream 1 ounce or so of brandy or liqueur Fudge sauce (if frosting on cake is sparse) 4 ounces whipping cream (optional)

One good serving of cake will make 4 desserts or more, depending on how you put it all together. Take 4 parfait glasses, and into each put a daub of ice cream, then a thin layer of cake, frosting and all (thin because the parfait is going to be stored in the freezer and you don't want the cake to be frozen when served). Sprinkle a few drops of the brandy over the cake. (The alcohol in the brandy and the sugar in the cake will, together, help keep the cake soft.) Add more ice cream, then another layer of cake and brandy, ending up with the ice cream. Freeze. When ready to serve, allow to stand out to the refrigerator for 3 or 4 minutes, in case the cake needs any further thawing. Douse with fudge sauce and whipped cream. For a final kick, add just a bit of the brandy to the cream before you've completed the whipping. CHEAPSKATE'S TRIFLE (4 servings)

Here's a variation on that old English dessert, the trifle. You can make it in parfait glasses, or in a large, deep dish. 1 large piece leftover pound cake 1 ounce or so of brandy or liquer 1/2 cup fresh fruit (strawberries and pineapple are recommended) 1 cup custard, warm 4 ounces whipping cream

Crumble the pound cake into a bowl and sprinkle (not drown) with brandy or liqueur. Distribute among four parfait glasses. Add a layer of fresh fruit. Pour warm custard over fruit. (Check any standard cookbook for a good custard recipe.) Refrigerate. When ready to serve, top with the freshly whipped cream. A little more brandy on top? Why not? DOGGY BAG BUCK DELIGHT

Finally, a recipe for duck soup. Certainly now that you're a full-fledged Doggy Bag Master, you not only laid claim to the duck carcass, but asked the waiter to bag your leftover moo shi dish as well. Aaaaah. Duck carcass Clove of garlic, crushed 1/4 teaspoon fresh ginger, minced Cellophane noodles, or other pasta Leftover moo shi pork, chicken or beef Sesame oil

Simmer the carcass in a large heavy pot with water to cover. The broth should simmer for at least 1 hour, the better to pull out every last bit of flavor from the duck. Add the garlic and ginger 5 minutes before the broth is finished (their flavors have a tendency to cook away).

When the broth has taken on character (you can almost guage it by the number of people who pop into the kitchen to find out what's cooking), remove from the heat. Strain the broth into a bowl, then back again into the pot, and put the carcass aside to cool. When it's cool enough to handle, remove the meat. Throw away bone and gristle, but save the skin for later. Cook the cellophane noodles in boiling water and drain.

Put the broth back on the heat. When it begins to bubble, add the moo shi leftovers, the noodles, and a drop or two of the sesame oil. When it starts to bubble again, it's ready. Serve instantly to be sure the vegetables retain their crispness.

Note: Once you've made the broth, instead of adding noodles and moo shi leftovers, you can go any way you like -- a good duck broth can be eaten as is, or it can lead you down a hundred different roads of delight. For example, simmer the duck with crushed garlic and lemon juice (a tablespoon of juice for each two cups of broth), and then, just before serving, throw in a cup of leftover rice. What about the skin? You can try a variation on a Jewish snack called grieven, made by scraping off any clinging fat and slowly baking at about 200 degrees, spooning off additional fat as it collects. When the skin is crisp but still a little chewy, sprinkle with salt, and it's ready. Eat as is, or use it to make a kosher Chinese BLT.