Herman Dorfman has been playing the piano at parties for 20 years. "I try to assess the mood of the party and the age of the people," he says. "I provide background and stimulation, sort of the like the booze."

Dorfman says "As Time Goes By" is the partygoers' favorite tune. "Lots of pretty women seem to like 'Send in the Clowns,"' he adds.

Another veteran piano player, "Doc" Traupel performs at over a hundred private parties each year. "You can't play disco on the piano," he says. "Most people want to hear Cole Porter or the current pop tunes, but I've never learned a song I haven't used somewhere."

A professional soloist can play anything for your party, from semi-classical background music to sing-a-longs or ragtime tunes. Booking a professional pianist through a top agent will cost you $100 to $150 for three hours or any fraction thereof. And if you don't have a piano, the pianist will bring along an electric one for an extra $20 or so.

Few woman pianists work the party circuit, so you can expect your piano player to show up dressed in a tuxedo. But if you like, some musicians will wear a costume that fits in with an ethnic celebration, holiday bash or kid's birthday.

Make arrangements several weeks in advance if you're planning to hire a piano player for a major do. According to Sidney Seidenman, a local agent, Saturdays are the busiest nights of the week, and December 1, 8 and 15 will be the most heavily booked nights of the year.

Maybe you can't afford the agent's price tag for a professional pianist. Or you can't find an agent who can cater to your particular taste. There are several options.

Check out the local talent yourself. The soloist in your favorite piano bar may be available on nights off. The orchestra pianist at a hotel reception may book soos as well. If you find a professional pianist you'd like to hire, ask for the name of his agent and call. This way you know in advance what you're getting.

If you're looking for free-form improvisational jazz, cruise restaurants and clubs where musicians are hungry enough to play for tips. Or check for a bulletin board where musicians' cards may be posted. f

Many music stores also provide space for musicians advertising for work. And it's always worth talking to someone: most free agents say they rely on word-of-mouth for their bookings.

At local universities, most music faculty-members are too busy to perform at parties, but for $10 or $20 an hour you can hire a student. Some music schools keep a list of performers and make referrals; others will post your request. Although music training usually emphasizes the classical, many students have diverse experience. And they've been pressed into service at enough family gatherings and school functions to master party favorites.

The only rule of thumb in hiring a piano player is don't settle for anything you don't really want. If you can't find a solo pianist to suit your taste or the mood of your party, crank up the Victrola.