NEW YORK (AP) _ Marlene Betterman and Delois Gregory, a pair of modern ''Avon ladies,'' say the old sales approach of simply picking a block and ringing doorbells doesn't work as well as it once did.

Fewer customers are home in the daytime. Fear of crime has made some people wary of opening their doors to strangers.

Today's ''Avon lady,'' ''Fuller Brush man'' and others like them are more likely to get their foot in the door by telephone, television or mail. The sales brochure and catalogs that can be passed around the office are among their most useful selling tools.

Direct sellers have been modernizing their sales techniques in recent years. They are hopeful the changes will spark industrywide sales that have been stuck near $8.6 billion for three years.

Mrs. Betterman has sold Avon products for 21 years in the rural communities near her home in tiny Garfield, Minn., 120 miles west of Minneapolis.

''When I started out, I could go almost any time of the day and any day of the week and find my ladies home,'' she said. ''Now, 75 percent of my consumers are working, pursuing careers on their own or getting more involved in church activities.''

She has adjusted by setting up appointments to meet customers on their days off, stopping by workplaces to prospect for new business and making her deliveries at night. She sold more than $20,000 worth of Avon products last year.

Meantime on the East Coast, Ms. Gregory refuses to make door-to-door calls alone in the tough Brooklyn, N.Y., neighborhood she calls home.

''Door-to-door is fine if people already know you,'' she said. ''But if you are new, people won't open their doors.''

Instead, she passes out brochures with her name and number on them at subway stations, stops by beauty parlors and medical centers to leave a catalog and takes orders from her co-workers at the Post Office where she works the overnight shift.

In only her third year with Avon, Ms. Gregory sold $73,500 worth of merchandise last year. Avon's 375,000 U.S. sales representatives generally earn commissions of about 45 percent of what they sell.

Direct sellers are usually in business for themselves, working as independent agents rather than as employees of their companies.

Their main attraction is individual attention and service. Unlike the frenzied retail sales clerk, the direct salesman has time to demonstrate how a product works and leaves his number in case there are problems.

They are distantly related to pioneer-day merchants who carried goods in horse-drawn wagons to communities that had neither the time nor the inclination to produce them.

Companies that sold door-to-door proliferated between the Great Depression and the start of World War II, said George C. Hescock, executive vice president of the Direct Selling Association, a Washington, D.C.-based trade group. Their sales forces were made up mostly of men who worked fulltime peddling cookware, books and household products.

After the war, door-to-door sales became a popular parttime occupation, Hescock said, and housewives gravitated to the business.

Today, the ''Fuller Brush man'' finds himself in the minority. About 70 percent of the 15,000 Fuller Brush dealers are women, the company said.

Avon Products, which celebrates its 100th birthday this year, and Fuller Brush, which turns 80, are perhaps the best-known door-to-door sellers, but they have been adding new twists to their approaches.

The New York-based Avon, for instance, has dropped its longstanding prohibition against selling in the workplace, a practice that it once felt would conflict with Avon representatives who sought customers at their homes.

''The trend toward women being in the workplace and not at home was a fundamental change, and we were probably late in recognizing that,'' said James Preston, president of the Avon Products division at Avon.

He said sales in the workplace account for about 25 percent of the division's $1.1 billion in domestic sales, up from 15 percent three years ago.

Avon also has tried to reach new customers via direct mailings and general advertising that list a toll-free number for customers who want to place an order. The company services the order and then alerts the appropriate Avon representative that they have someone in their area who is interested in the Avon line, thereby expanding their agent's base of customers.

Fuller Brush, which sells a range of inexpensive household products, encourages its dealers to call ahead and make appointments before going door- to-door, said Derek J. Stryker, vice president of Fuller Brush's household division in North Kansas City, Mo. The company declined to disclose sales figures, saying that is the policy of its parent, the Chicago-based Sara Lee Corp.

Lesser-known door-to-door sellers are making adjustments as well.

Watkins Inc., a privately held concern based in Winona, Minn., has sold an assortment of household, food and health products door-to-door since 1868.

But today, it derives only about 20 percent of its sales from the approach, said Richard Wantock, president. He also would not specify sales figures.

He said the company began encouraging its sales agents in the 1970s to hold demonstration parties where the products could be shown to a group of people at once. That approach has been used for years by Tupperware Home Parties, the Orlando, Fla.-based concern that sells plastic products, and Mary Kay Cosmetics Inc., a beauty products company based in Dallas.

A few door-to-door purists continue to thrive.

Southwestern Co., based in Nashville, Tenn., sells books the same way it did when it was founded in 1855, spokesman Jim Simpson said.

Each summer, the company signs contracts with college students who are trained and then sent in teams to different parts of the country for 12 weeks of door-to-door selling. Simpson said Southwestern recruited about 3,000 students for the job this past summer. The privately held company does not release sales figures, but they are believed to exceed $15 million.

Electrolux Corp., the Stamford, Conn.-based seller of vacuum cleaners, rug shampooers and floor polishers, also continues to rely heavily on ''cold canvassing,'' going door-to-door without advance notice.

''What we are selling has to be demonstrated in the home,'' said John Zauner, vice president of sales administration. ''We want the customer to see how it works and give them a chance to operate it themselves.''

Like Fuller Brush, Electrolux is a Sara Lee subsidiary and does not release its sales figures. It employs 20,000 people nationwide.

The Direct Selling Association estimates that direct selling revenue grew 63 percent to $8.5 billion in the eight years ended in 1982. But sales have since been flat, holding at $8.5 billion in 1983 before edging up to $8.6 billion in 1984, the association said.

Traditional businesses that have begun offering parttime jobs and flexible hours have made it more difficult for direct sellers to find sales agents. That has meant trouble for direct sellers that fueled sales growth by expanding the sales force.

Joseph Kozloff, who follows Avon for the investment firm of PaineWebber, said some of these companies are now trying to help their salesmen become more productive.

Avon, for instance, is testing how to tailor its sales catalogs to regional tastes. If the company finds jewelry sells better in one region, for example, it may insert a few extra pages of jewelry items in catalogs used in that region.

Advertising also is getting a boost.

Avon tried to nurture its image by spending $22 million on advertising last year and plans to increase it 23 percent to $27 million this year, Preston said.

Encyclopaedia Britannica USA, the educational publisher based in Chicago, is spending about $20 million on advertising, said David Van Tosh, executive vice president for national sales.

Tupperware Home Parties is spending $15 million to advertise its lineup of plastic houseware, ovenware and educational toys, said Jack Linn, vice president for sales training at company headquarters in Orlando, Fla.

End Adv Sunday Sept 14