CAMBRIDGE, Mass. (AP) _ Rising tuition costs are making college selection a major financial decision, and scores of independent advisers are now competing for a share of a growing market: parents and students willing to pay $500 or more for guidance through the admissions maze.

''With tuitions amounting to $16,000 a year times four, you want to be sure to spend it in the right place,'' says William B. Pierce, executive director of the Independent Educational Counselors Association, a Cape Cod-based national organization that has grown since 1976 to more than 90 full-time, private counselors.

''The problem is so many choices out there,'' says Rosalyn S. Lowenhaupt, an IECA member in St. Louis. ''People need guidance. Some people, especially those who don't have it for themselves, are trying to buy an education as a gift for their children.''

Others caution that private counselors, those not associated with schools, are no guarantee for success.

''A lot of people go to independent counselors believing erroneously that independent counselors can provide them with that extra bit of savvy to get them into that extra competitive college,'' says David C. Erdmann, admissions dean at Rollins College in Winter Park, Fla.

He heads the National College Counseling Project that has discovered that high school seniors spend only an average of 20 minutes with college counselors in school.

Many seniors are learning this week which colleges have accepted them. For many schools, mid-April is the deadline for notifying applicants.

Until two years ago, the 3,300-member National Association of College Admissions Counselors in Skokie, Ill., would not admit as full members advisers unassociated with schools. That changed as budget cutbacks cost some members their jobs, said executive director Charles Marshal. About 50 members now operate their own advising services.

Both associations guard professional counseling standards and are wary of what Pierce called ''the moonlighters.''

Most full-time professionals depend on discreet referrals for clients. Counselors such as William Colom, 39, of Stoneham, use newspaper ads: ''Is the admission process confusing you? RELAX.''

Colom, a guidance counselor for the Cambridge schools, runs his admissions service as a sideline.

Lynn Meyer-Gay insists a prepared part-timer can provide one-on-one service unavailable from overburdened school guidance counselors.

''If you're sending your kid to a reasonably competitive school, you're paying $40,000 to $60,000 for four years,'' she said. ''It's an investment a lot of people are making with less than a half hour of someone's expertise.''

Mrs. Meyer-Gay's first client was her daughter, Kristen, now 19, who had to overcome prep school grades that, because of the learning disability dyslexia, her mother feared did not reflect her intelligence.

''She deserved the best learning environment,'' said Mrs. Meyer-Gay, ''and I was determined to get it for her.''

Kristen had assembled a working model waterfall in the family's living room as part of her hobby, working with plants.

Mrs. Meyer-Gay included glossy photographs of the waterfall and of other work in a slick 10-page brochure she assembled to accompany Kristen's applications to six schools, each chosen as being open to her daughter's interest in plants, animals and ceramic sculpture.

In her admissions request packet, Mrs. Meyer-Gay included a letter advising admissions deans of Kristen's dyslexia. She said her daughter was highly creative and able to overcome her difficulty in manipulating numbers and letters.

Cornell accepted Kristen, and she has since made the dean's list and is working toward a degree in landscape architecture.

''The waterfall did it,'' says her mother. ''They got a picture of the whole person. It clearly earmarked her as somebody whom they wanted.''

Inspired by success, Mrs. Meyer-Gay and Angelica Sawyer, a neighbor who shepherded four daughters through college admissions, now operate College Bound, a business advising students seeking higher learning.

Neither is a professional counselor. Mrs. Sawyer has served as an advocate for learning disabled students, and Mrs. Meyer-Gay is a former assistant professor of architecture at North Carolina State University.

They weigh students' aspirations and qualifications against school requirements, help them fill out applications for admission and financial aid, even edit the essays that most good schools require.

''Be specific, absolutely concrete about your achievements,'' advises Mrs. Meyer-Gay, mentioning an applicant who simply listed swimming as his sport when he was a regional champion.

One client wanted to study civil engineering but thought he might switch to architecture or medicine. His Scholastic Aptitude Test scores were 50 to 75 points below the level most engineering schools accepted.

After 25 hours of catalog thumbing, Mrs. Meyer-Gay suggested Princeton, Rensselaer Polytechnic Institute, and Carnegie-Mellon University. in Pittsburgh, schools that offer five-year programs combining arts and sciences and engineering.

College Bound charges about $25 an hour, with flat fees of $500 for uncomplicated cases.

William F. Elliot, vice president of enrollment at Carnegie-Mellon says independent counselors fill a "human need" and that he regularly asks their help in tracking choice students for his school.

"In some cases, they raise aspirations of kids, in some kids they raise anxieties," he said. "The real good kids are going to survive, in spite of parents and in spite of counselors.