Maybe it has to do with New Year's resolutions. Or maybe the cold weather has people looking for better ways to keep warm. Whatever the reasons, therapists at Washington's Human Sexuality Institute report an increase in the number of persons seeking help since the beginning of the year.

With every grocery store crammed with sexual self-help material these days, it may be hard to believe that people are still searching for answers to sexual problems. But they are.

Since Masters and Johnson gave birth to the new specialty 12 years ago, sex therapy has become big business. Clinics have sprung up around the country, and the number of practitioners has risen steadily. Today, about 1,200 therapists are "certified" by the American Association of Sex Educators, Counselors and Therapists, and countless others serve the field.

Several journals are published regularly with titles like Human Sexuality, The Journal of Sex Research and Sexology Today. There is even a Society for the Scientific Study of Sex.

As sex therapy has grown, cases have become more difficult to solve, not easier. One reason for this, of course, is the fact that there is such a wealth of information available on sex.

"People have educated themselves," explains Marguerite Fogel, a therapist at the Human Sexuality Institute, whose average number of five calls a week has been about doubled lately. "They've taken care of the simpler problems." Also, straightforward problems often are handled now by family doctors, leaving the more complicated cases for the specialists.

One big complaint is "lack of desire." Up to 40 percent of persons who seek help at sex centers do so for that reason. It's not that "lack of desire" is on the increase, say the experts. It's that more people seem to feel the need to do something about it. Because these cases may involve deep-seated anxieties and conflicts, they often are difficult to treat.

"Lack of desire is a symptom that a person's energy is going into other psychological problems he may be unaware of," says Dr. Julius Fogel, medical director of the Human Sexuality Institute. Says Robert Phillips, another Institute therapist, "You're often talking about heavy-duty marriage counseling here."

Another major problem being brought to sex therapists is phobic avoidance. In such cases, sufferers become tense and panic-stricken at the mere thought of sexual activity. Sometimes they will sweat profusely or become nauseated. Because they've often undergone a traumatic sexual experience in the past, phobic patients are often treated with a combination of behavioral and psychoanalytic techniques.

Associated with Helen Singer Kaplan of New York Hospital-Cornell Medical Center, this so-called "psycho-sexual" approach represents a major modification of the treatment format first described by William Masters and Virginia Johnson. Then, the two pioneers avoided lengthy talks about childhood conflicts and unconscious interference in favor of rapid behavioral treatment. But today, because of the difficult nature of some cases, some therapists are finding it necessary to use a more eclectic approach.

Other more logistical details of the original prescription have been modified as well.

"We had hoped that people would modify, elaborate and develop research designs themselves," Virginia Johnson has said. "We don't have the only game in town."

Most therapists outside the Masters and Johnson Institute now see clients once a week rather than daily. And although domestic pressures may be distracting, clients usually practice their behavioral exercises at home, instead of retreating to the neutral setting of a hotel, as recommended by Masters and Johnson.

Masters and Johnson also recommend that male and female therapists work together as teams, under the theory that each partner will have someone to identify with. But studies have shown that one therapist is as effective as two.

"In our experience, when a therapist is sensitive, well trained, and experienced . . . then he or she can effectively conduct sex therapy on a solo basis," says Kaplan in her book, The New Sex Therapy. Therefore, many therapists have chosen to work individually, saving clients a double expense.

New treatment may involve the use of special instructional aids, such as gender dolls and videotapes. At the Human Sexuality Institute, specially made films are used to demonstrate such exercises as the "stop-and-go" technique used in treating premature ejaculation.

Others give instruction on foreplay and positioning, plus general information about sexual anatomy. "The films are particularly useful," says institute therapist Cara Hoffert, "because clients suddenly realize that other people have exactly the same problem they have. It helps them not to feel so isolated."

Just how effective is sex therapy?

In 1980, two California sex therapists, Bernie Zilbergeld and Michael Evans, caused quite a commotion in sexology circles by sharply criticizing the high efficacy claims of Masters and Johnson, published originally in Human Sexual Inadequacy (1970).

After a thorough review of the book, which they found laced with "methodological errors and slipshod reporting," Zilbergeld and Evans concluded in a cover story for Psychology Today: " . . . The fact is that the evidence for the effectiveness of Masters' and Johnson's therapy -- and therefore that of almost all sex therapy -- is less solid than has generally been believed."

New York psychiatrist Thomas Szasz, perhaps best known for his book, The Myth of Mental Illness, believes the idea behind sex therapy is highly misleading. In his book Sex by Prescription (1980), Szasz argues that the creation of medical labels for sexual problems -- like "erectile dysfunction" -- implies they are diseases in the same way that, say, cancer of the colon is.

Turning sex into a medical specialty, Szasz says, removes it from the realm of personal values and encourages the notion that doctors are experts on sexual pleasure. "Every educated person knows that if he wants to learn about good eating," Szasz writes, "he must consult Craig Claiborne, not his gastroenterologist . . . "

Therapists themselves acknowledge their techniques can sometimes degenerate into over-simplified gymnastics and are acutely sensitive to the problems inherent in trying to change such private behavior. "If you only know the correct exercise to give," says therapist Phillips, "you aren't going to get very far."

As cases have become more recalcitrant, some therapists have become more realistic in their expectations of success. A good number believe the early days of the sure-fire behaviorist cures are over. Or some, like HSI director Fogel, believe "there never were any easy cases."

Still, many people who seek sex therapy find that it works for them. And new figures released by the Masters and Johnson Institute continue to show a success rate of 80 percent.

In the end, any discussion of sex therapy's efficacy must deal with the question of how to define success. Sometimes a couple's sexual performance will improve technically, while satisfaction declines. Or, conversely, performance remains the same, while the relationship grows richer.

At the Human Sexuality Institute, staffers believe more people go away satisfied than dissatisfied.

"Our aim," says Phillips, "is to help people feel better about themselves and each other. If divorce happens, that's okay. The mark of success is whether they feel they have been able to make the best decision for themselves."