THE WORST thing that can happen on a picnic is to arrive at the site and find out you've forgotten the can opener, napkins, or knife.

If you picnic often, you might as well keep a container filled with the items you feel essential. If you picnic regularly you might want to have a special place for the "almost absolute necessities" as well.

Here are some suggestions for what to pack, how to pack and how to keep foods from spoiling, followed by an eminently portable picnic menu. ABSOLUTE NECESSITIES

Almost every picnic requires most of them: salt and pepper, bug repellant, wash-and-dry towels, can and bottle openers, napkins. ALMOST ABSOLUTE NECESSITIES

Plates and cups, forks, spoons and knives, tablecloth (cloth or heavy plastic or tatami matting) with anchors to fight the breeze, wide-mouth insulated containers for carrying ice, disposable foil grills, beach chairs. NOT NECESSITIES, BUT A GREAT HELP

To picnic simply, the following items are helpful:

Insulated containers which today are so inexpensive that one can afford several varieties.

The Styrofoam carriers, also very cheap, will hold the food that should be kept cold or hot.

Insulated bottles and jars will hold the liquids.

Ice packs that are frozen ahead and resuable keep anything cold for a short period of time.

In a pinch, many thicknesses of newspaper are good insulators. PREVENTING SPOILAGE

It is absolutely essential to keep foods from spoiling, unless you want your picnic to end up with a trek to the hospital. Holding perishables at room temperature is dangerous to health. The trick is to keep hot foods above 140 degrees and cold foods below 40 degrees.

Some items can be carried frozen to the picnic site and will thaw on the way.

Some dishes can be heated at home, carried hot and if further reheating is required, this can be done on top of a portable grill or over an alcohol burner. HELPFUL HINTS

Little tricks learned along the way may save you much time and energy:

Deviled eggs, synonymous with picnics; can be carried more easily in an egg carton.

Bar cookies and some cakes ride well in their original baking pans.

When packing the picnic basket, wrap individual place settings of flatware in a napkin or two and secure with a colorful pipe cleaner.

Pack the basket in reverse order of unloading, that is, the tablecloth should be on top.

Keep the carriers of cold food out of direct sunlight.

Items such as smoked meats, cold fried chicken, breads, and unfilled pastries are less susceptible to spoilage and may be carried without benefit of insulating container. NANCY'S CHICKEN AND RICE SALAD 2 cups raw rice, cooked one day ahead and refrigerated 3 cups cooked, diced chicken or turkey 1 cup slivered almonds 1/2 cup minced onion 1 1/2 cups yogurt 3 tablespoons curry powder 1/4 cup soy sauce 2 tablespoons vinegar

Combine cooked rice, chicken or turkey, almonds, onion. In a separate bowl mix yogurt, curry powder, soy sauce and vinegar. Mix the rice-chicken mixture with the dressing and chill thoroughly. Decorate with pimentos to serve. DILLY BEANS (6 to 8 servings) 6 to 8 cups fresh green beans Salt 1/2 cup white vinegar 1/2 cup sugar 1/2 cup water 1 teaspoon dried dill weed or 1 tablespoon fresh

Cook green beans in boiling water until tender crisp. Rinse under cold water to stop cooking process. Season with salt. While beans are cooling, combine vinegar, sugar, water and dill weed. Mix and pour over beans. Marinate at least overnight. Serve chilled. GLAZED CHOCOLATE CHIP CAKE 2 1/4 cups sifted cake flour 1 cup sugar 2 teaspoons baking powder 1 teaspoon salt 1/2 cup butter or margarine 1 teaspoon vanilla 5 egg yolks 3/4 cup milk 2 squares unsweetened chocolate 1/2 cup chopped pecans 1/2 cup orange juice 1 cup confectioners' sugar

Sift flour, sugar, baking powder and salt together. Add butter, vanilla, egg yolks and 1/2 cup milk and beat 2 minutes. Add remaining milk and beat 2 more minutes. Grate chocolate. Add pecans and grated chocolate to batter. 2Pour mixture into greased and floured 9-by-5-by-3 inch loaf pan and bake at 350 degrees for 60 to 70 minutes. Cool in pan.

Mix orange juice and confectioners' sugar. Spoon mixture over top of warm cake. Pierce surface of cake with fork to allow glaze to penetrate. Freeze if desired after cake is cool. POTTED CHEDDAR 2 cups (8 ounces) shredded sharp cheddar cheese 2 tablespoons butter 2 tablespoons port wine 1/4 teaspoon paprika

Blend all ingredients together until smooth. Refrigerate or freeze. Serve, cold but not frozen with sesame seed crackers.