Washington — A few years ago, Dorothea Johnson - former etiquette writer for the Washington Star and entertainment lecturer for the Foreign Service Institute and elsewhere - received an invitation to ''A Party.''
''It said, 'Regrets only,' but I phoned the hostess anyway to ask her what kind of party it was,'' Mrs. Johnson recounted at a recent lecture, ''and she said, 'Oh, just a party.' I asked her, what should I wear, and she said, 'Oh, anything.' So I asked, 'What are you wearing?' ''
'' 'Oh, I haven't given it a thought,' '' came the reply.
Mrs. Johnson got out her ''I'm-not-sure dress,'' she says, which is a basic black, ''and my husband saw it and got out his I'm-not-sure suit - a three-piece gray.'' They arrived at the party only to discover everyone else there dressed more casually, ''except a liaison from the British Embassy who said, 'I say, aren't we overdressed?' ''
Hostesses should be kinder to their guests, says Mrs. Johnson, a trim, manicured veteran of countless parties here and in the 20 other posts where her Marine family has been stationed.
Invitations should let guests know a lot more than where and when the party will occur, she thinks. Mrs. Johnson usually states the nature of the party (dinner, brunch, etc.) and gives the guests some idea of who else will be there (''You know the Daniels, don't you? And we're having a couple who just moved here from Maine''), how much food to expect (appetizers or a full-course dinner) , and an idea of what the host and hostess will be wearing.
All of this information should be issued by phone, she says. ''I lecture to people in the diplomatic corps, and all over the world hostesses are having trouble with invitations. I haven't sent out an invitation in eight years.''
Instead, she gets a yes or no by phone, and then follows up with a reminder in the mail about a week before the party, giving the same details, and including a map for newcomers. She issues the telephone invitation one to two months in advance because ''around here, you have to get your bid in early.''
Then she gets out her calendar and breaks down the preparation for the party into several steps, doing as much as possible in the days and weeks before the party occurs. ''I try to do almost nothing the day of the party, so I can be relaxed for my guests,'' she says, calling this ''the key to a good party. If you're relaxed, your guests will be relaxed and will leave your place happy.''
It's easier to stay relaxed if you start with ''the kind of party with which you're most comfortable,'' she says. ''You don't need to copy your neighbor or your friend, although I'm not above stealing a good idea here and there.''
She also emphasizes that the food you prepare ''is not the key to a successful party. If the food is splendid or elaborate, but you're a nervous wreck, what do you think your guests will remember?'' Simple food that can be prepared in advance and heated up quickly does best, and no more than three courses are really necessary (main dish, salad, and dessert).
You should also check with your guests before planning the menu, especially if you plan to serve foods that may be taboo to many - pork, seafood, foods cooked or soaked with liquor. Mrs. Johnson avoids serving fowl with bones (''I don't think guests should have to struggle with it'') and anything that requires cutting, if the party is a buffet. ''I've seen too many accidents happen when people hold their plates on their laps and try to cut,'' she says.
Some Washington hostesses she interviewed for her column confessed that they use the same menus every year, ''and it doesn't bother them. They say that people don't come for the food anyway.''
Here are more of Mrs. Johnson's ''party props'':
* Pink lightbulbs. ''These make the women look beautiful and the men look handsome, and hide the dust,'' she says.
* Tablecloths. She makes them out of inexpensive cotton or sheets on sale, and protects them from dripping wax by charring her candles before the party (lighting them, blowing them out, and pinching off the charred wick). She also wards against spills from hot drinks by filling cups no more than three-quarters full - and serving them in the living room, if possible.
A last safety precaution is to set the table first with service plates - large, glass plates that remain on the table throughout the changes of course. ''I was once complimented on my fine Swedish crystal plates until I just couldn't stand it,'' she says, ''and had to confess that they're Pyrex pizza plates I bought for $1.29 each from the PX (post exchange).''
* Menus. Mrs. Johnson writes out a copy on anything from informal note paper to paper bags (for a picnic), and places them between the place settings, although ''the White House places them across the plate.''
* Seating. She makes up a chart ahead of time and uses place cards, because she believes that saying, ''Oh, just sit anywhere,'' means you don't care. Another Washington hostess uses place cards as a conversation piece, she reports. ''She makes up clever names for each of her guests,'' Mrs. Johnson explains. ''I'm always Emily Post Jr. A retired Naval officer was The Old Man of the Sea, and one fellow - a really shy man - was The Swinger,'' titles that guests feel they have to live up to for the rest of the evening.
* Guests of honor. Honoring a special friend is a great excuse for a party, says Mrs. Johnson. Remember that an honored male guest sits on the hostess's right and an honored female guest to the right of the host. The place of honor in the living room, she says, is the right-hand side of the couch - a place that more than one unfortunate member of the diplomatic corps has unknowingly usurped , she reports.
* Hors d'oeuvres. Mrs. Johnson doesn't serve these anymore, substituting a soup course that she serves in cups while her guests are still in the living room.
* Buffet. A clever way to bring people to the buffet line is to slip the men names of women they are to escort, she says, or draw seasonal decorations (Christmas trees, shamrocks), cut them in patterns down the middle, and have the two genders match up the halves. ''That way married couples don't always sit together.''
Mrs. Johnson also advises hostesses to try sitting everywhere they expect their guests to sit (including the floor) to see if it's comfortable - and reliable (''I've seen chairs break on guests''), and serving beverages from a tray after everyone's been served their food rather than have people balance a plate, silverware, napkin, and glass as they wander about looking for a place to sit.
* Conversation. She advises using Eleanor Roosevelt's trick with the alphabet: ''She'd start with A and think, Apples - Aren't the apples lovely this year? If that bombs, she'd go to B, Books - Has anyone read any good books lately? One night, they say, she got home and told FDR about a terrible evening - she'd gotten all the way to W!''
She also helps ensure good conversation by never inviting only people from the same office or occupation (''we've never had only military''), and inviting ''at least one new couple'' to every party.
* Working women. She's seen some splendid parties done all with carryout food , ''and nobody knew but the hostess and me.'' By setting the table the night before and coming home early enough to dump the carryout into pots and scoop ice cream into fancy dishes, you can pull off an elegant party - without having to ask your guests to help.