THE LADY standing next to me at the meat counter watched as I chose a standing rib roast. "My family likes roast beef," she said, "But I just can't get it to come out rare."

She isn't alone in this complaint. A juicy slice of roast beef, a leg of lamb, still pink and moist in the center, or a roasting hen, well browned and crisp on the outside but running with juices on the inside, are some of the simplest and most appealing dishes in a cook's repertoire. Despite this simplicity, many people aren't sure how to achieve these results, and nobody wants to risk ruining an expensive roast. Yet roasting is the easiest of cooking techniques, once you know the rules.

Roasting is a dry-heat method for cooking meats (although vegetables are also roasted sometimes) in which a large piece of meat is placed in an open pan in a moderately hot oven. The outside of the meat should be well browned and flavorful and the inside juicy, the degree of doneness depending on the type of meat and the cook's preference.

Not every cut of meat can be roasted successfully. In general, only tender cuts that are well marbled with fat should be considered. Prime and choice grades of beef have enough fat if the cut is a tender one--rib roast, rump, boneless chuck and tenderloin--but the lower grades of beef should be braised or stewed instead. All poultry, including game birds, does well in the oven because the juices are sealed in by the skin. With veal, any of the large cuts (over four pounds) may be roasted except the shanks and breast. However, the high price of veal has made it a rarity in the display case, and you may have to special-order it from your butcher. The abundance of fat in pork makes it a good choice for roasting. And lamb is tender enough that all large cuts take well to this cooking method. Keeping Meat Juicy

When a piece of meat is tender but is not well marbled, the cook must add some fat in one of several ways. Fat may be placed on the outside of the roast--either by covering the meat with a thin sheet of pork fat, marinating it in oil or smearing soft butter on the exterior.

Other methods of artificially marbling meat call for inserting fat into the roast rather than applying it to the outside. One method, called larding, is somewhat more difficult and involved than injecting, but is a common practice in Europe where the beef is much leaner than American meat. Long thin strips of fat, which may have been marinated in herbs, garlic or brandy, are threaded through slits in the meat with the help of a larding needle. The procedure requires practice and a good larding needle.

A more modern method uses a culinary hypodermic syringe to inject fat into the meat. Turkey producers have been doing this for years and calling their products "butter-basted." You may butter-baste your own turkey or any other cut of meat by injecting a mixture of two parts butter to one part vegetable oil. The vegetable oil keeps the butter from solidifying in the needle of the syringe as you inject the cold meat. While you can use all vegetable oil or even a liquid margarine for the baste, I prefer butter because it adds to the flavor.

Finally, the simplest method of all is to baste the meat with the fat that has accumulated in the bottom of the roasting pan several times as it cooks. This is most easily done with a bulb baster. Getting Ready to Roast

In general, any fat on the surface of the meat is left on, so that the roast bastes itself. The exceptions to this rule are venison, lamb or mutton, since many people object to the strong taste of the fat in these meats. If you do, use a small sharp knife to strip away every bit of fat. You will find that lamb has a very delicate flavor, much like veal or poultry, when cooked without its fat or fell.

Seasoning your roast is a very individual matter. A beef roast may need no more than a little salt and pepper. Yet some cuts benefit from a heavier hand with the herbs and spices. They can be added in several different ways. Salt, pepper, herbs, spices, garlic, dijon mustard and seasoning sauces, such as soy or worcestershire, may be rubbed or brushed over the outside, though their flavors will not penetrate the roast entirely. On the other hand, marinades add considerable flavor, and when the seasoned meat is allowed to sit for a long time, the flavors do penetrate nearly to the center of the roast. A good example is sauerbraten--a German beef roast marinated from one to three weeks in wine, vinegar and herbs. As the sauerbraten sits, it develops a strong spicy flavor. With pork and lamb, many chefs like to cut small deep incisions in the meat and to insert slivers of garlic so that the roast is well flavored with it.

As your roast cooks, juices and fat will flow out of the meat into the bottom of the roasting pan. These juices caramelize and become the base for your gravy. However, they may have a poor effect on the bottom of your roast, causing it to stew or fry rather than roast. The bottom of the roasting pan (along with the accumulated juices and fat) conducts more heat to the bottom of your roast than the air in the oven, causing it to cook faster. To avoid this, all roasts and poultry should be placed on a rack in the roasting pan. These small racks are available in cookware stores. But a better method is to use a bed of coarsely chopped vegetables. This serves the same purpose as the rack and has the advantage of adding flavor to the gravy. Onions, carrots, celery, mushroom stems, shallots, leeks or parsley may be used. Choose those vegetables that will add a flavor you like to the gravy. It's also good to add a bay leaf and a pinch of thyme or other herbs to your bed of vegetables. You may economize and use the stems of parsley, trimmings of leeks or the ends of carrots. This is quite acceptable if they are still fresh tasting.

Poultry and boned meat must be trussed for even cooking. The string holds the meat together firmly so the whole roast cooks evenly. Boned roasts purchased in grocery stores are already tied or placed in elastic net. Poultry, however, must be trussed at home. Although they can be cooked without trussing, birds are tastier and more moist when the legs and wings are secured tightly against the body. There are so many ways to truss a bird that I suggest you consult Jacque Pepin's books "La Technique" and "La Methode," in which he gives step-by-step instructions with photographs to demonstrate three or four methods. Other basic cookbooks, such as "The Joy of Cooking" and Julia Child's "Mastering the Art of French Cooking," give directions on trussing illustrated with sketches. Temperature, Timing, Tools

Now the roast is ready for the oven, but the oven must be ready for the roast. It should have a thermostat that may be set with a fair degree of accuracy. If you are in doubt, buy an oven thermometer. In fact, every kitchen should have one. The shelf should be placed in the center of the oven or just below. Convection ovens--those with the heat source outside the cavity of the oven and a blower that forces hot air into the cavity--should not be used for roasting. Although they do roast evenly, the forced air dries out the roast and leaves it less juicy.

Two other pieces of equipment will help you on your quest for the perfect roast--a bulb baster and a meat thermometer. The best bulb baster is stainless steel with replaceable rubber bulbs. There are two types of meat thermometers, the kind that is inserted in the roast before it goes in the oven and the instant reading type that does not go in the oven. I like the instant reading thermometers because they may be used for other foods, and their accuracy is easily tested by placing in boiling water. The instant reading type also doesn't leave a hole in the meat. You may also test several parts of the roast to see if the center is rare and the ends are medium so that the roast suits several members of the family.

The most frequent mistakes made in cooking roasts is covering the pan with a lid and/or adding water to the pan. Either of these will result in braising or stewing the meat. Either one will make it impossible to produce a rare roast or even a medium roast. The steam created will cook the meat all the way through and make it seem drier and less tender.

Both temperature and timing are very important in roasting. I suggest that you use a reliable cookbook that has charts with times and temperatures for the various types of meat. Use these charts as guidelines, but use a meat thermometer to tell you exactly when your roast is done.

The internal temperatures for beef are 125 degrees for rare, 135 degrees for medium, and 150 degrees for well done. The internal temperatures for lamb are 135 degrees for rare, 145 degrees for medium and 155 for well done. Poultry is cooked to an internal temperature of 165 degrees. To measure a bird's temperature, insert the thermometer in the the thigh as well as in the thickest part of the breast. Pork must always be cooked until the internal temperature is at least 170 degrees in order to destroy trichinae organisms. Veal develops more flavor if it is roasted slowly until it's internal temperature reaches 160 degrees. If you don't have access to a chart for oven temperatures and times, you can use this general rule of thumb: You may start any roast at 425 degrees for 10 minutes to sear it, then reduce the temperature to 325 degrees and cook until done. The exception is beef tenderloin and any other very tender, boneless, fat-free roast. Instead, these are roasted at 450 degrees for a very short time so that they don't dry out. A medium-size tenderloin, for instance, will cook to the rare stage in 25 minutes.

There are several tests for doneness other than internal temperature, which is the most accurate. In poultry, veal and pork, cook until the juices run clear and are no longer pink. If you are cooking a large roast, a deep slash with a knife will be necessary to tell how the center is doing. In all meat, one can test for springiness by pushing a finger into the surface of the meat, although this method requires some experience. As the meat cooks, it becomes softer and less springy. In addition, French chefs often pierce the roast with a metal skewer, leave it in for about 10 seconds and then hold the skewer against their wrist. With practice, one can tell by judging the heat in the metal. And certainly if it's still cold, you'll know the roast isn't done at all. You may want to test both of these methods in combination with the thermometer until you feel confident of your own judgment.

Carving your roast is an art in itself. Gone are the days when the man of the house prided himself in his tableside manner and skill with a carving knife. Again, I would recommend Jacque Pepin's books for step-by-step advice on carving different types of roasts.

Your roast will carve more easily if you allow it to rest for 10 minutes after it comes out of the oven. This allows the juices to return to the outer, more-cooked parts.

To review the main rules for roasting, remember these points.

* Choose a tender cut of meat that is well marbled with fat, or add extra fat to a lean, tender roast through larding, basting, injecting with butter or covering with fat.

* Season the meat to your taste by coating with herbs, marinating or inserting slivers of garlic.

* Boned roasts must be tied and poultry may be trussed for more even cooking.

* Place the meat on a rack or a bed of vegetables to keep the meat up and out of the juices and fat.

* Never cover the pan or add water to it.

* Follow a chart for roasting time and oven temperature, and then cook until the internal temperature indicates that the roast is done to your liking.

* When the roast is done, remove from the oven and allow it to rest for at least 10 minutes before carving.

* To make gravy, add some water or other liquid, such as wine, to the roasting pan to dissolve the caramelized meat juices. Strain to remove the vegetables. Season liquid to taste and bring to a boil. Thicken with a little cornstarch or arrowroot dissolved in water, or with a beurre manie (equal parts butter and flour rubbed together then added to the liquid). ITALIAN-STYLE BONED LEG OF LAMB (8 servings) 4 to 5 pound leg of lamb, boned 1/4 cup olive oil 2 tablespoons of minced Italian parsley 2 cloves garlic, minced Grated peel of 1 lemon Pinch of cloves 1 teaspoon freshly ground black pepper 1 teaspoon salt 1 teaspoon dried sage 1 tablespoon rosemary

Have the butcher bone and tie the leg of lamb. Mix the remaining ingredients and coat the lamb with them. Let rest in the refrigerator, covered, overnight. Place the lamb on a rack in a roasting pan. Preheat oven to 450 degrees. Place lamb in the oven. After 10 minutes reduce heat to 350 degrees and continue baking for about 1 1/2 hours. Use the thermometer to test for doneness, 135 degrees for rare and 145 for medium. ROAST OF VEAL WITH DIJONAISE SAUCE (6 to 8 servings) 4 pound veal rump or round roast 6 ounces of pork fat, cut in thin slices Salt and pepper to taste 1 onion, thickly sliced 1 cup of carrot slices 4 shallots, sliced 1 clove garlic, mashed Pinch of thyme 1 bay leaf 1/4 cup white wine 2 tablespoons dijon-style mustard 1 cup creme fraiche or heavy cream

Place the thin slices of pork fat over the veal roast. Sprinkle with salt and pepper. Prepare a bed of vegetables with the onion, carrots, shallots, garlic, thyme and bay leaf. Place the roast on the bed of vegetables. Preheat the oven to 425 degrees. Reduce the temperature to 325 degrees when you place the roast in the oven. Cook at 325 degrees for about 25 minutes per pound. Begin testing with the thermometer after an hour and 15 minutes. The veal is done when the internal temperature reaches 160 degrees. There will be no pink juices when pierced with a skewer. Remove from the oven when done. Remove the veal from the roasting pan, and let it rest for 10 minutes. Meanwhile make the sauce. Pour the white wine over the vegetables in the pan over medium heat. Stir up all the browned bits. Pour the contents of the roasting pan into a strainer. Press the juices out of the vegetables using a wooden spoon. Add the dijon mustard to the pan juices. Stir the cre'me fraiche into the dijon mixture. Adjust the seasoning.