If you've ever wondered how to get a foot in the stage door, consider the case of Jeremy Mayer. Since last fall, Jeremy has starred in several school productions in Arlington, a local TV special called "Matthew's Magical Christmas" (he was Matthew), and the Ford's Theatre production of "Orphans' Revenge," in which he played a dying orphan and a flying angel.

Jeremy, 14, picked up the acting techniques for his new career at Stage Door, a summer workshop for 10- through 16-year-old actors sponsored by the Children's Theatre of Arlington.

The three-week course for beginning and advanced actors includes improvisation techniques, oral interpretation, acting skills, dance, mime, makeup, set design and stage lighting, and culminates in a variety show.

Not all its graduates get into Actors' Equity, as Jeremy has, although many have starred in local community theater and school productions, says spokeswoman Marcy Ubois.

That's the good news.

The bad news is this summer's course already is filled "beyond capacity, though we do like to have the names of those who are interested, to use in our other CTA productions," Ubois says. If you or your child falls into that category, call the Children's Theatre at 558-2161.

But several drama programs in Northern Virginia still have openings this summer.

One is the Followspot Theatre Camp in Fairfax County, where Stage Door's director, Susan McInerney, will teach improvisation. McInerney is one of five full-time staff members who will be part of the program this summer, sponsored by the county's Performing and Fine Arts Department.

The camp also will include part-time staff members who will teach specific workshops. Among the staff members will be a mime artist from Archaesus Productions Inc. in the District, a choreographer who teaches at the University of Maryland, a juggler, a puppeteer, a clown, a costume designer and a makeup artist.

But most of the 9- to-17-year-old campers, program organizers suspect, will cluster around Mike Baker, a video specialist from WNVT-TV (Channel 53), who will teach on-the-spot newscasting. If the results are good enough, he promises, the station will air the students' efforts later this year.

The camp, at Jackson Intermediate School, will last two weeks, says recreation specialist Cindy Bailey, with the kids rotating through the various offerings the first week and concentrating on their favorite spots the second.

Although the program is open to any child, priority will go to Fairfax County residents.

McLean Community Center is offering two programs this summer: one for third- through eighth-grade graduates and another, more expensive, one for older students.

The program for the younger group is called Import/Export, where students will explore "world folk tales" through "creative drama." The program is being directed by Gay Hill, head of a local troupe called The Amalgamated Children's Works and former director of Glen Echo's Adventure Theatre. She promises to teach sensory awareness, characterization, improvisation, story dramatization, speech skills, sets, lights, music, mime, movement, costumes and masks during the four-week session. McLean residents will get priority in participating in the program, officials say.

The McLean Community Center also is offering Teen Theatre Workshop, the most professional and most expensive of the various summer programs.

All applicants must audition for the four-week program.

Acting will be taught in master classes by Arena Stage veterans Richard Bauer and Halo Wines. Set design will be taught by John Lee Beatty, who won a Tony in 1980 for his set designs for the Broadway production of "Tally's Folly."

Participants will receive a makeup kit and professional instruction in its use from Bob Kelly, who runs Bob Kelly Makeup Inc. in New York City. Kelly designed Elizabeth Taylor's wigs for "The Little Foxes," and has been involved extensively in television, films and Broadway. Dance instruction will come from Liz Lerman, artistic director of the Dance Exchange in Washington, and directing classes will be taught by Bob Alexander, director of the Arena's Living Stage.

Students will attend two professional shows and will produce one of their own, "No Mother to Guide Her," described as an old-fashioned melodrama.

The program is headed by John Duncan, drama director at McLean High School, and Joel Snyder, performing arts director at the community center.

Like the community center's program for younger students, the workshop will give first priority to McLean residents.

Alexandria residents are going to have to settle for being last on the McLean or Fairfax lists, since the city offers no children's theater, and Mount Vernon, which has a very good children's theater, is dark this summer. "We have no building of our own," explains spokeswoman Bobbie Erb, "and we found last summer that by the time we paid for rent and air conditioning, we'd eaten up our funds. Catch us again next fall."

Here are the details on the camps still open:

* Followspot Theatre Camp, Fairfax. June 22 through July 2, Tuesdays through Fridays, 9 a.m. to 3 p.m. at Jackson Intermediate School, Falls Church. Ages 9-17 admitted, with priority going to Fairfax residents. Cost: $75 per student, which covers tuition, miscellaneous supplies and T-shirt. Call 691-2671 for more information.

* Import-Export, a Creative Dramatics Workshop, McLean. June 28 through July 23, Mondays through Fridays, 9:30 a.m. to noon at the McLean Community Center. Graduates of third through eighth grade admitted, with priority going to McLean residents. Cost: Before June 4, $75 for residents of the Dranesville tax district; $100 for nonresidents; after June 4, $85 for tax district residents, and $110 for nonresidents. Call 790-9248 for more information.

* Teen Theatre Workshop, McLean. July 6 through July 31, 9:30 a.m. to 4 p.m. at the McLean Community Center. Eighth grade through recent high school graduates must apply and audition by June 10, with priority going to residents. Cost: $195 for Dranesville tax district residents; $225 for nonresidents. Call 790-9248 for more information.