ON ST. Valentine's Day, everyone has special license to be conventional. Considering how most people strain, the rest of the year, to be original, creative, funny, shocking, unpredictable and individualistic, this should come as a relief.

On Valentine's Day, you do not even have to be true to yourself. It is so much more amusing to promise to be true to another.

Hiding a piece of jewelry in a bouquet of flowers is considered to be the height of cleverness on Valentine's Day. The height of wit is to say, "Will you be my valentine?" Giving one rose and saying "I love you" is also acceptable. Those who have never tried being quite that straight may be surprised to find how eagerly accepted they are.

Anything much above or below these heights is unacceptable. One does not play jokes that expose another person's sentimental gullibility. Miss Manners need hardly say that one does not send obscene valentines.

And especially, one does not explain or qualify declarations of admiration or love. A great deal is said between supposed lovers, these days, about wanting space and seeing if things work out and fulfilling needs that used to be more succinctly expressed as: Be mine.

It is not that Miss Manners is unaware that the course of love requires occasional adjusting. She has noticed, however, that anticipatory haggling doesn't seem to make it the slightest bit smoother, from which it would seem that the world could be spared a great deal of unattractive blathering. At least on Valentine's Day, let us try to be unstinting to our loved ones.

For those who can't imagine how to be conventional, here is a small guide to Valentine's Day.

CARDS

The valentine itself is one of the few greeting cards of which Miss Manners approves. Ordinarily, she feels that any statement that can be bought pre-printed is better expressed in one's own hand. But a valentine is something different from a letter declaring love. It is less specific, because it does not go on, as love letters inevitably do, to explain what it means by love. Between a sender expressing a fond glow and a recipient ready for all-consuming flame of passion, there is that marvelous ambiguity (which the latter always attributes to the shyness of the former). It is not, therefore, considered binding. Valentines are rarely produced as evidence in court.

The proper valentine has at least one, and often all, of the following ingredients: A red heart, paper lace, pictures of flowers and the word "love."

PRESENTS

Nothing with an electric cord or that requires batteries is a proper Valentine's Day present. The traditional present is a pair of gloves, but any or all of the following are also permissible: A diamond ring, a heart-shaped box of candy, a diamond bracelet, a handkerchief, a nosegay, a bottle of perfume or toilet water, a diamond necklace, a book of poetry or a kiss. You will notice that many of these items are appropriate for either gender, but traditionally, gentlemen are the only bestowers of diamonds, and ladies of kisses. On Valentine's Day, above all times, it is the thought that counts.

REMARKS

"You have beautiful eyes," "I never imagined anyone like you could really exist," "I've never felt this way before," etc.

A final word: If you find all this ridiculous, repetitive, unoriginal and silly, please remember that the same can be said of more advanced forms of love making -- but only by outsiders. Oddly enough, those who are doing and saying what lovers have done and said since the world began are never bored. Dare to be the same.

MISS MANNERS RESPONDS

Q. I have infrequent phone conversations with my out-of-state sister-in-law. I have just deduced that certain clicking noises from her end of the telephone are a tape recorder.

She, as yet, does not realize that I realize what's going on. How can I tactfully handle this abhorrent situation?

A. We have, here, a rare case in which the more tact you used, the worse you would make the situation.

Suppose, for example, you began a subtle discussion of the laws and customs concerning recording telephone conversations. Graciously you might concede that one party has the right to record, but that the unilateral enforcement of such a right might make the other person uncomfortable.

You could even become charmingly apologetic about saying that you, for example, are made nervous by the idea, just as some people prefer not to have their photographs taken although they cast no aspersions on those who enjoy taking photographs.

Can you see how thoroughly you would outrage your sister-in-law by showing that you think she's a sneak and a snoop but are too nice to say so?

Suppose, instead, you took the tactless approach: "Good heavens, what's that clicking? That's not a tape recorder, is it? Don't tell me you're recording all these conversations? Oh, no, I'll never be able to say another word, if I know you're taping it."

She need only reply, "I'm sorry, it never occurred to me it would bother you," and that would be the end of it.

You may consider this a great curiosity in the realm of etiquette.

Q. I will soon be celebrating my 50th wedding anniversary, and my wife is still constantly jealous! In all these years, I have never been unfaithful -- and I don't know many young people today who could say as much -- but she is still always after me.

Since we retired, we don't bother to cook any more, and eat all of our meals out, in a nearby cafeteria or small neighborhood restaurants. Nothing fancy, as we are on a limited income, although comfortable enough compared to many of our friends. We always did like to eat out, though, even when we were busier. Naturally, we see the same faces all the time in these places, and I like to be friendly, so I kid around with the girls or tell them when they look sharp.

That's all I do, though, but it's enough for my wife, Flo, to make a big deal out of. For years I just ignored her, figuring that if it wasn't that, she'd find something else to fuss over, but since our last move, two years ago, I noticed something different. Some of the girls at the restaurants we go to have acted angry or made put-down remarks to me, when I was only trying to be a gallant gentleman. I got really angry when I thought Flo had put them up to this, but she denied that and said that times have changed and young girls don't like that any more. Is this true? I never said anything dirty, only complimentary. Is the age of gallantry dead?

A. First, congratulations on your anniversary. Miss Manners is sure that you are quite right in believing that few young people can lay claim to 50 years of fidelity. She also congratulates you and your wife on your fidelity to this dispute. To maintain the same conflicting positions at 50 years worth of meals, without either side yielding or escalating it into a major conflict, is an impressive feat of endurance.

You inquire if the age of gallantry is dead. Let us first examine what gallantry traditionally meant, in the relationship between a gentleman and a person employed to wait upon him.

As you may recall from childhood, people who waited on table were treated as invisible until comparatively recent times. Nowadays, many people, such as yourself, consider this to be snobbish, and prefer to exchange pleasantries with them. These democrats run the risk of falling into the snobbery of assuming that their social attentions must be welcome to the help.

There was a tradition, if one may dignify it by that name, that a gentleman had the right to expect young women in his service to be flattered by his personal attentions. Whether this took the form of pinching the parlormaid on the backstairs or worse, it was never respectable, and the gentleman knew it, which is why he took care not to be observed by the lady of the house. Your compliments, however innocently intended, derive from this tradition. Consider, if you doubt this, how you would feel if your wife told the busboy she thought him handsome, or asked for the mailman's opinion of how she looked in a particular dress.

The fact is that people who are performing their jobs have a right to consider themselves immune from personal appraisal by the customers. This is far from meaning that they all care to exercise this right. Many women, as you know, enjoy having their looks commented upon, providing, of course, that the assessment is flattering. But an increasing number of women feel that such compliments -- implying that one of their functions is to be pleasant-looking to the customers -- are insulting.

Miss Manners would not characterize your wife's objections as jealousy, but as an objection to a transgression of taste. She hopes that this discussion is not depriving you of an issue that has provided 50 years of dinner-time entertainment.