Every evening for the last three months, MetroVision salesman James Gilkesson has grabbed his subscription forms and brochure, piled into his battered BMW, and taken to the streets of Prince George's County to sell cable television door-to-door.

Gilkesson is marketing the communications system of tomorrow with the sales techniques of yesteryear.

"They've done it every way," said Winfield Kelly, the one-time Prince George's politician-turned-corporate-executive with the county's other cable franchise holder, Storer Cable Communications of Maryland, which also is sending salesmen door to door.

The two cable companies have found "that the only way you can explain to people and have them understand you is to have salesmen going door to door," Kelly said.

The streets serving more than 36,000 homes in Prince George's County already have been wired for cable, but the residents cannot expect to get their commercial-free movies, round-the-clock sports or all-country music stations until at least two to four months after the construction crews have gone. First, a cable television man such as Gilkesson must knock on their doors, and they must sign up.

In this slow process, only about 8,700 homes have been reached by salesmen from both companies so far. About 6,500 have subscribed.

The pace in Prince George's matches that of every other phase in the development of cable in the county. It has been seven years since the county first began to wrestle with a cable television ordinance and two years since two franchises were awarded after pitched political battles among companies competing for the coveted franchises.

Some 13,000 residents in many municipalities, such as Hyattsville and New Carrollton, which granted franchises earlier, already have cable in their homes. But in areas outside the municipalities, the sales of cable is just getting under way. The county's two franchise holders, Storer in the northern half and MetroVision in the south, recently completed the first year of their three-year building program that ultimately will cover the county, meaning everyone who lives on streets in those designated first-year construction areas now have access to cable..

Today, the cable trunk lines in the county's southern half fan out from the headquarters of MetroVision of Prince George's, on Ritchie Road, where the company gets as many as 100 calls a day from eager county residents.

"They see that cable running down the street and they pick up the phone and say, 'Where's my cable?' " said Roger Wells, MetroVision's general manager.

Most of the callers are disappointed to find out they may have to wait as long as two years if they live in areas slated for the third year of the building program, he said.

But they may take comfort in the fact that no one is exempt from the long delay. Howard Stone, executive director of the county's cable commission, is still waiting for a MetroVision salesman to reach his Largo condominium complex.

Meanwhile, Gilkesson is visiting homes as quickly as he can.

"This is the best door-to-door sales job you could have," he said during a recent sales trip to the New Orchard Estates subdivision near Largo. "You're selling a product that you like, that people like and one that they don't have."

The most persuasive allies in selling cable, Gilkesson has learned, are the children of the prospective customers.

Last week, when Gilkesson pulled into the parking lot of the Centennial Village town houses in the Landover area, 15-year-old Terry Downey was the first person to come running.

For days, Downey had been trying to get his mother together with Gilkesson to talk. This time he fetched his mother from a neighbor's home.

After a brief discussion, Peggy Downey pulled out her checkbook.

She said she feels a bit guilty about not taking her son to the movies more often, and is getting the service mainly for him. She has never seen cable television before, but now plans to take a look. "Believe me, for my $34 a month, I will check it out," she said. "Money is tight."

Earlier, in Ruby Bryson's living room, Gilkesson went through his well-rehearsed presentation, flipping through the pages of a green looseleaf notebook and talking about the 19 satellite channels his company offers.

Bryson asked only two questions about the bewildering choice of services available, ranging in cost from $2.60 to $32.95 for a package with three movie channels and a large assortment of other satellite offerings.

It wasn't long before 12-year-old daughter Daphne had produced her mother's checkbook and handed it to Bryson.

Bryson ordered the works, she said, mainly for her children. Her 14-year-old son wants to watch sports, Daphne loves movies and 6-year-old Melanie wants MTV, which offers 24-hour-a-day videotaped rock.

MetroVision sells 53 percent of the homes its salesmen visit, giving it what is considered a high penetration rate in the industry, according to cable experts.

Younger families and sports fans are the easiest to sell, Gilkesson said, and senior citizens put up the most resistance.

"The first thing you have to do is get them to see that we don't have any X-rated movies," said Gilkesson. "Once you break them of that, it's okay."

Nationally, cable salesmen spend an average of about six minutes in each home they visit. But in Prince George's, salesmen are averaging 25 minutes a home, according to the staff of the county's cable commission.

That's because residents of the Washington area have little experience with cable, since most areas are still in the early stages of the long cable process.

And so Gilkesson continues his painstaking, door-to-door search for customers. "That's my job," he said. "And it's a lot better than selling insurance or vacuum cleaners."