You sang "Auld Lang Syne" hours ago, it seems, but your New Year's Eve blast is such a hit that several guests, drinks in hand, still show no signs of leaving.

You, of course, are delighted everyone's had so much fun, but it's been a long day. You're ready to call it quits.

Now for the ultimate test of the complete host or hostess: How do you get the hangers-on to go home? Diplomatically. Without offending friends and neighbors. And without resorting to playing your old "Bells Are Ringing" album -- the one with Judy Holliday singing several choruses of "The party's over, it's time to call it a day."

Getting that last guest out the door can be a problem at almost any kind of party -- from the big cocktail reception to a little at-home dinner -- agree some socially prominent Washingtonians.

At least a couple suggest that something stronger than diplomatic hints may be necessary.

Joan Braden, who gave a book party at her home recently for Henry Kissinger, says she alerted guests that time was up by asking entertainer Danny Kaye to put his fingers to hs mouth in a shrill whistle.

That proved effective, says Braden, project coordinator for the American Petroleum Institute. But, of course, you can't always count on Kaye showing up at your party.

And on that particular night Kaye's whistle turned out to be somewhat premature. (A warning to any overanxious host or hostess in Washington, where delays at the office are endemic.)

The Bradens, Kaye andKissinger, who all had been invited out to dinner following the book party, were leaving the Bradens' house when Senate Majority Leader Robert Byrd of West Virginia strolled up the sidewalk. He, Sen. Jacob Javits (R-N.Y.)) and other senators had been delayed by a late session.

Not wanting to slam the door in the Senate leader's face -- though the party was over -- the Bradens resolved the crisis quickly. Tom Braden (the columnist) and Kissinger stayed on for awhile to entertain Byrd and the other senators while Joan Braden and Kaye kept the dinner appointment.

At a big party he gave six years ago, recalls Ridgewell's Caterers president Jeffrey Ellis, the clock had struck 3 and still about 30 of the guests stayed on. "I was tired, and I had to get up at 7 a.m. "This is enough foolishness,' I told myself." So at 4, "I just went upstairs to go to sleep."

Did that end the party? Not quite.

"My guests threw me in a cold shower." Only then, he says, did they go home.

Ellis, whose firm caters many a lavish reception, says one of the most effective ways to end a party "is to run out of booze."

The more some people drink, the harder it is to get them to go, especially those with "the gift of gab" who settle down as if "they're in the corner bar." Don't make it too comfortable, he advises.

Another effective trick is to serve coffee at evening's end. "That makes them think it's the end of the party."

If you've got close friends in the group, Ellis says, you could get them to start suggesting to the other guests that the hour is late and it's time to go.

What he doesn't advise if "turnng the lights on and off," the traumatizing technique favored by many bartenders eager to lock up at 2 a.m.

Lucy Moorhead, author of "Entertaining in Washington," agrees that capping the bottles at a party "tends to get the message across in a subtle way. It's the only tried and true way."

The catch here s that in many friendly get-togethers -- particularly where there is no bartender -- the guests may feel so much at home that they may be pouring their own drinks. In that case, she suggests "standing up and staying on your feet."

If no one catches on, you go a bit farther and remark casually that "you've got a heavy day tomorrow."

She recalls a dinner party she attended years ago where the host (whose name she declined to reveal) "was eager to get the guests to leave. He ran upstairs and put on his pajamas."

The guests apparently took no offense. "They all started to roar with laughter and then went home."

Moorhead, whose husband is the longtime Democratic congressman from Pennsylvania, notes that in recent years many people have expressed their dislike for cocktail parties. But, she says, "I don't see why they are in such ill repute. If people find them too noisy, they can go home."

To her, the cocktail party -- often given for a specified period, say from 6 to 8 p.m. -- is a valuable form of entertaining, especially in politics. f"It's a way to have a lot of people in. You can be in and out in 15 minutes. "I think it's here to stay and serves a useful purpose."

Socialite Ina Ginsburg, chairman of the Fans of the American Film Institute, says she doesn't "happen to enjoy" giving cocktail parties, but she did hold one recently -- maybe her only one in 20 years -- at the Fairfax Hotel in honor of artist Andy Warhol. When the hour grew late and only stragglers remained, she, the guest of honor and hotel owner John Coleman simply "went upstairs." That ended that party.

Though Susan Hurley, social secretary at the United Arab Emirates, says stragglers are not a problem at the diplocatic parties she helps the ambassador give, the reason may be that they don't expect the quests to go home on time.

The invitations say 6 to 8 p.m. or 7 to 9 p.m., but "you expect people to stay until 10 or later," she says. The ambassador usually remains until 10. When he leaves, "the other people leave."

Val Cook, who keeps abreast of the social circuit as fashion director for Saks-Jandel, thinks party diehards are less of a problem in official Washington than in other cities such as New York.

"It's an interesting phenomenon in Washington," she says. "Here people are so concerned about the job they do that almost everybody leaves by 11:30 or a quarter to 12."

One Washington hostess says she gave up giving afternoon cocktail parties that seemed to stretch on and on because she worried about guests driving home if they had had one too many. Instead, she gives brunches,"which are more finite." And she dilutes the Bloody Marys in later rounds.

If it happens that your guests Monday night aren't ambassadors and cabinet secretaries and they ignore all your diplomatic hints, a young Washington wife has a final technique that works for at least half of her family:

"I excuse myself and go to bed and leave my husband to do all the talking."