When a Christmas gift becomes a hot potato it's time to call on the arbiters of American etiquette for advice.

Letitia Baldrige, Charlotte Ford, Elizabeth Post and Judith Martin (Miss Manners) are in unusual agreement on that most difficult of all gifts - the one you don't want to keep."Don't keep it," is their unanimous advice. "And don't feel guilty about it."

They even agree that you can pass such a gift on to a friend next Christmas without breaking any of the rules of correct behavior.

"Just be sure to label the gift as to the giver so you don't risk passing it back to him or her," advised Baldrige, whose books on business etiquette are a must for corporate executives.

"I did that once with a piece of crystal given to me by a friend in Chicago, and she was nice enough not to say anything to me, but I learned of my faux pas from a mutual friend and have been trying to make up for it ever since."

Exchanging an unwanted gift at its store of origin for another item or for credit is the usual practice, and the giver needn't be told, unless it's just a matter of getting a different size. However, if the giver lives at some distance, the gift may have to be returned to him for the exchange.

"Even if you don't keep a gift, send a thank-you note anyway and don't let on it is not going to be used," suggested Elizabeth Post, who updates the Emily Post etiquette books originated by her grandmother-in-law.

"Only if it is a piece of clothing given by a very close friend who might notice you never wear it need you have to explain."

You can of course keep the unwanted gift and bring it out of the closet only when the giver is visiting you.

"You don't want to hurt the feelings of dear old Aunt Jane, who gave you an ugly vase, so bring it out when she visits," Post said. "It's not being hypocritical. It's good etiquette."

"It's really too time consuming to take unwanted gifts back," pointed out motor heiress Ford, whose etiquette books are aimed at younger Americans. "I put them in a closet for a year and eventually pass them on to someone else who might like them. The only thing you can't give away is something that's been handmade personally for you."

Thanks for even a dreadful gift should be couched in terms of "surprise, pleasure, and gratitude," according to Martin.

"Whether you plan to cherish it, exchange it or throw it away is irrelevant to the manner in which you accept it. In the case of an unwanted present, you must never put the burden back on the giver by letting on, and what makes this impossible is another rule that prevents the giver from asking, `Why don't you use what I gave you?' "

A problem of another sort arises when someone with whom you don't usually exchange Christmas gifts gives you a present. What if you don't wish to reciprocate?

"You should never feel obligated to run out and get a gift to exchange," said Post. "A thank-you note is enough or perhaps a little remembrance at New Year's. Next year get off an early Christmas card that will indicate you are not going to begin a gift exchange."

"Send a gracious thank-you note and don't mention an exchange," said Baldrige. "Gift giving does not have to be reciprocal. Sometimes people just want to thank you for some particular favor you have done them in the preceding year.

"And then there is that sleazy, favor-currying type of gift that people send to get in good with you. This warrants a cool thank-you note, not a warm, enthusiastic one. If you're lucky, the giver will get the message."

Ford said that in her experience, "If you get a gift and just send a thank-you note, believe me you won't get another gift next year."

All the authorities pointed out that thank-you notes are preferable to thank-you telephone calls.

"Calls are terribly intrusive and always come when I'm in the bathtub or taking a souffle out of the oven," said Baldrige. A thank-you note is something to cherish, and if it's clever, something to show around. It makes an enormous impression these days because so few people take the time to write."

"I definitely think thanks should be written," chimed Ford. "If a friend has taken the time to go out and shop for a gift, you should have time to write a few lines."

Post said that ending a gift exchange, even one of many years standing, need not be difficult.

"You simply suggest it, but be sure to do it during the year and not just before Christmas. Call and say you want to cut down on your list, that your family has grown so much you just can't afford to give so many gifts to friends. When you ask `Would you mind not exchanging?' you will find the other party is usually delighted."

"It's happened to me and I've said `Great!' " said Ford. We all get bogged down with too many gifts. You can always send a contribution to charity in the other person's name as a way of getting out of gift shopping."

It's harder to stop exchanging gifts with family members, but it can be suggested "if nobody's nose gets out of joint," Ford added.

Another potential hot potato is the gift given by an employer to an employee. Need the employee reciprocate?

"Absolutely not," said the etiquette experts.

"Only if a personal relationship exists between two persons working for the same firm need there be any reciprocation," said Post. "When a boss gives his secretary a present, she is not obligated to reciprocate."

"It's actually embarrassing for an employer when an employee gives him a gift," said Baldrige. "To give a boss a gift might be misinterpreted as a bribe for special favors.

"Only if a secretary has a special relationship with her boss's family might she want to give her boss something for his hobby and something for the family, possibly homemade, like cookies."

Baldrige touched on an even more sensitive gift problem "out of sympathy for young people who are having a hard time making a go of it."

"Sometimes, if you have rich relatives, it's all right to say you'd rather have a check than a gift. But it can only be done if you're young and in a bind financially."