About 20,000 people are employed on Capitol Hill, or that's true until the day after an election, when defeated incumbents' staffers suddenly find themselves scrambling to find a new slot.

Few people can be as lucky as Kerry Dumbaugh. Not 24 hours off the Greyhound bus from Butler, Pa., on a snowy January morning in 1977, she strode into her senator's office to introduce herself to "a friend of a friend" who worked there. Soon after, she walked out the door as a bonafide "staff assistant."

As it turned out, she had -- quite by accident -- showed up at an opportune time. Sen. John Heinz (R-Pa.), elected only two months earlier, was moving into his new office and needed help. One of Dumbaugh's first duties was "unpacking boxes." But it was the foot in the door she needed to begin a career in Washington.

The upheaval of arrivals and departures in official Washington can be a good time to latch on to a Capitol Hill job, as Dumbaugh discovered. Also, during any year an estimated 8,000 vacancies occur in congressional positions--which means there are jobs to be had for those savvy enough to find them.

Dumbaugh, 31, now chief legislative assitant for Rep. Jack Hightower (D-Tex.), has teamed up with Gary Serota, 31, executive director of the Congressional Management Foundation, to write a short, snappy guidebook to finding a job on Capitol Hill. Called Capital Jobs (Tilden Press, 108 pp., $5.95), it is aimed at offering their "insiders" advice to job-seekers unfamiliar with congressional hiring practices.

Since each of the 435 House members and 100 senators hires independently, the first step, say the authors, is to target only 15 or 20 offices you might want to work for, rather than diluting efforts by trying to contact everybody. To narrow the choice, begin with members of Congress who share your political philosophy (in the long run, you probably will be happier in your work) or those who are interested in a subject in which you have expertise.

Once you have determined your targets, getting a job with one of them, the authors acknowledge, often is a matter of "who you know," though they qualify that quickly by adding, "You don't have to know them well." What the Capitol Hill job-hunter needs is something that makes his or her resume' stand out from the hundreds that pour in to every congressional office.

Their suggestion: Get your network of family, friends and acquaintances moving in your behalf. "Do you have parents, brothers, sisters, grandparents, aunts, uncles, in-laws, distant cousins--anyone at all who will go to bat for you, who might be a hotshot in the community?" If your aunt is active in home-town politics, get her to write a letter (or have the mayor write it) in your behalf.

Other advice:

* Try to present yourself in a "service mode," as someone offering assistance or providing information.

* Learn to speak in Capitol Hill jargon. The book provides a short glossary of "Hillspeak." S.O.B., for example, is "not what you think. It stands for Senate Office Building."

* Be persistent in your job hunt. In most offices, "the expectation is that you will lobby. They're not turned off by persistence. But don't be a pest."

* Demonstrate "political sensitivity." Somebody who can anticipate home-town reaction is much in demand.

* Be specific about the job you want, which means doing the advance research to find out what is available. Make a "cold call" to someone in the office and ask. Better yet, get any Capitol Hill friends to keep their eyes open for you.

* Establish a "personal, first-name relationship" with the offices that interest you. Hand-carry your letter and resume' instead of mailing it, but don't linger after the delivery.

* Consider a part-time job or an internship to get started.

Insiders have the best chance at the good openings, claim the authors, but outsiders can and do break in. While a new session of Congress brings many staff changes, another big turnover period is about 90 days after freshmen legislators move into their new offices.

Write Dumbaugh and Serota: "By mid- to late-April of such years, scores of staffers who came to Washington from the 'home districts' after a successful election campaign have returned home, disappointed to find that answering constituent mail, paying high Washington rents and working long, long hours isn't the glamorous congressional life style they expected to find." Get started now laying the groundwork, they advise, for those spring openings.

For many like Dumbaugh and Serota, however, the rewards of a Hill job outweigh the drawbacks. Even in the first year of the job, "you think you are forging national policy," they say. "You can have access to the dream-makers and make recommendations to the decision-makers." It's that kind of heady environment that keeps the job-seekers lining up for interviews.

Kerry Dumbaugh and Gary Serota will offer a free seminar for "off-the-Hill" applicants looking for a job on Capitol Hill at 4 p.m. Nov. 30, Room 2362, Rayburn House Office Building. For more information: Capitol Jobs Seminar, 1737 DeSales St. NW, Suite 300, Washington, D.C. 20036.