A good saleswoman knows her customers, and Elizabeth Brinton, 13, knows hers -- the young people like Samoas, older ones prefer Thin Mints and single guys like oatmeal or peanut butter.

That's why Elizabeth, of Fairfax County, already has sold 6,100 boxes of Girl Scout cookies so far this season, while the average Girl Scout has sold 89.

Yesterday afternoon, just like every afternoon during cookie season (Feb. 22 to March 31), Elizabeth rushed home from school, loaded 17 cases of Thin Mints, Tag-Alongs and others into her family's car and set up shop at the Ballston Metro station by 3:30 p.m.

"The secret is to push, push, push," said Elizabeth, an eighth grader at Glasgow Intermediate School. "You have to hawk. You really have to push the cookies a lot. And look the people in the eye."

Although she normally sells at Ballston or, on rainy days, at the Crystal City Metro station, Elizabeth said she helped out last Saturday at the Girl Scout Cookie Store at 18th and M streets.

The most cookies any scout has ever sold at the cookie store is 111 boxes; Elizabeth, who belongs to Troop 2387, sold 213.

"We think she's phenomenal," said Jacqueline Browne, spokeswoman for the Girl Scout Council of the Nation's Capital. "She sells six or eight boxes at a time, not one or two."

"She makes people feel guilty, which is not something I want to see in print, but it's the truth," said Elizabeth's mother, Noel. "If it's cold, she'll switch her lines to 'Buy the coldest cookie in town.' When it was windy, and the bus at Ballston crushed two boxes of Chocolate Chunks that blew away, she told people it was 'The windiest cookie in town.' "

And she hits the military hard. Not long ago, Elizabeth couldn't tell a Navy captain from an Army colonel. Now, she knows the uniforms, and she knows the ranks. "She'll say 'We go with the Navy,' " her mother said. "Or, 'Girl Scouts fly with the Air Force.' People smile . . . and they buy."

Elizabeth, whose interests range from riflery to stamp collecting, hopes to become a doctor, journalist, social worker or a veterinarian one day. Those occupations require, among other skills she said, good salesmanship.

Elizabeth made her most dramatic sale about a week ago, when a customer came by for a box of Samoas, and she sold her a case. "The woman is probably still wondering how she got a case of Samoas," said Noel Brinton.

Yesterday, Elizabeth persuaded Theresa Ledwith, 25, a training specialist for the disabled, to buy a box of cookies by appealing to Ledwith's German shepherd. "Dogs like cookies, too," she called out.

For every box of cookies, which cost $2.25, the scouts get to keep 36 cents. Elizabeth hopes to make enough to not only benefit her troop but to send needy Girl Scouts to camp and on field trips. Her original goal for the cookie season was 8,000.

"But, she's getting pretty perturbed about that kid in New York," said Elizabeth's mother, referring to 13-year-old Markita Andrews, billed by The New York Times as the greatest cookie seller of all time, who had, by the beginning of March, sold 2,300 boxes. "Elizabeth wants to beat her -- royally," her mother said.

And Elizabeth has another goal: to sell a box to President Reagan. "I think he'd like Samoas.