There is only one "must" about giving a dance. The music -- whether live orchestra, a single piano player, a juke box or just your own record player -- has got to be good. And that means good dancing music -- because dancing music has to have an up beat that will lift the spirit and make the toes start tapping.

It's always easy to find slow songs with which to alternate, but if you have too many of them the mood of the party changes and slows down accordingly. The real pros, like the late Meyer Davis, know how to orchestrate the mood of a party, playing a few fast songs to set the mood, then alternating with slower dance tunes. But as the evening goes on, particularly in the last hour, it's a good idea to heighten the intensity of the beat from song to song so that by the last half hour the guests are whirling around the floor hoping the music will never stop.

If you plan on live music get the best if you can -- Michael Carney or Peter Duchin, top orchestra leaders who play all over the country. But most goodsized cities have local orchestras which are good or a cocktail bar with a jazzy piano player. Just be sure you listen to the band ahead of time -- audition them if you have to -- then you'll know what you're getting.

Have as many pieces as you can afford, whether one piano player, a three-piece combo, or a 24-piece orchestra. Don't get a second-rate group because you'll save a hundred dollars. You can cut out flowers (borrow plants instead), skip elaborate food for supper, but you can't do anything if your orchestra comes in, sets itself up, and instead of a lifting sweep of sound produces a somnambulant beat. You've lost the point of having a dance.

"Continuous music" is another detail that must be negotiated ahead of time. If you don't contract for what's known in the trade as "continuous" -- this means that the orchestra will take no breaks for as long as you have contracted for the music to play -- you'll find that just as your guests are beginning to perk up and get themselves out onto the dance floor, the orchestra will decide to take a break, cutting right into the mood being developed and deflating the party like a punctured balloon. To pump the party up again will be progressively more difficult as your musicians periodically drop by the wayside and go wandering off to get themselves a drink or to sit in a heap mopping their brows and giving off waves of exhaustion.

If you don't have the top man himself there, it's possible that the players will try to take a break or two anyway. You must be firm in reminding them that you have contracted for continuous music and they must stay on the job. It is more expensive, but not that much, perhaps 15 percent, and it makes all the difference.

Prices on all this are impossible to predict -- it depends on too many variables -- how many pieces, whether the orchestra is local or coming in from a distance, how well known they are. Just ask what they charge.

It's a good idea also to ask beforehand how much it will be if you decide not to close the music at 1 in the morning or whatever hour you contracted for, but to keep the orchestra going a bit longer. Then you won't be floored the following day by a much larger bill than anticipated.