IT IS some years now since Miss Manners made it a rule never to attend social events at which she would be expected to stand up. Thus, Miss Manners misses out on a lot of good conversations about how awful cocktail parties are, because she doesn't go to cocktail parties, which is where these conversations are held.

There are other advantages to cocktail parties, of course. They are an excellent opportunity to meet all the people your friends don't like well enough to invite to dinner. And where else do you get a chance to eat a whole meal of anchovies and raw cauliflower?

However, this is no excuse for the trick of inviting people to dinner and then treating them, instead, to a long cocktail party followed by a midnight supper.

Such an event is usually scheduled to begin at 7:30 or 8. Drinks and finger food are served, but taken moderately, for the first hour, by guest who have been led to believe that they will soon be plied with fork food and wine. After that time, some of the smarter ones begin to understand that they have been duped and begin to go after the celery leaves.

Meanwhile, the hosts aren't budging. Naturally not. They have been eating themselves silly all day, under the pretext of sampling and tasting their cooking, and they just want to relax quietly with a few drinks until beddybye.

The excuse hosts give for this outrageous behavior is that they can't plan to serve dinner on time because their guests won't show up on time. Guests who have caught on to this then arrive later and later, and so the error is perpetuated.

It is the hosts who must correct this situation. It was their idea to stage the event, luring people from their own kitchens. And the guests are helpless - it is impossible to pack a sandwich in an evening purse.

They might begin by issuing a warning with the invitation. "We eat rather promptly," might be ominiously added when the time of the dinner is stated. The cocktail hour must then be severely limited to the hour from which it gets its name. Forty-five minutes would be better. Each prompt guest should have time for one and a half drinks and three canapes. Then dinner should be served, while everyone is still young enough to enjoy it.

What if guests haven't arrived by then? Well, the old rule was that you wait 15 minutes after the appointed hour for a man, and 20 for a woman. In the interests of equality, we might make it 17 1/2 minutes for each. And if some persist in being later, the rest of you may split the resulting extra deserts. MISS MANNERS RESPONDS

Q: What is the proper conduct of demonstrators at White House demonstrations, and of recipients (targets) of demonstrations?

A: The first obligation of the demonstrator is to be legible. Miss Manners cannot sympathize with a cause whose signs she cannot make out even with her glasses on. The most usual breach of conduct after that is that demonstrators vent their discontent on passersby, whom they should be impressing with their goodwill and reasonableness in order to suggest that it is the recipients of the demonstration who are crazy, not they. As for the recipients, the proper thing is to resist the temptation to look out of a White House window. Peeking is a mistake when one may wish to declare, the next day, that one was unaware of the demonstration.

Q: I am very interested in a young gentlemen engineer, but am unable to hold a non-technical conversation with him. Should I obtain a B.S. degree?

A: You see all those 20-year-old marriages that are breaking up all around you? Well, those wives are from an era when women educated themselves to be able to understand and talk intelligently about their husbands' careers. The husbands are now leaving them - or they are leaving the husbands - for someone who gives them a fresh new outlook on life.

Q: My big brother keeps listening in on my telephone conversations. I know he means well and is trying to help me, but it surely is annoying. Should I tell him to "bug off brother" or be more polite and suggest he be more inventive in his curiousity and good intentions?

A: It is, indeed, difficult to say such a thing to a beloved relative. Tell him Miss Manners says, "Bug off, brother." Unless, of course, you are speaking metaphorically about Big Brother, in which case you may have an interesting law suit.