Almost three of every four people who start exercising quit within nine months to a year. Yet, the results of six new studies suggest how exercise can become a lifetime habit.

Three factors are key to long-term exercise maintenance, report University of Mississippi Medical Center researchers in the Journal of Consulting and Clinical Psychology:

* Working out with others.

* Setting flexible goals.

* Distracting yourself when exercise becomes tiring.

Psychologist John E. Martin and his colleagues examined 143 "apparently healthy" but sedentary adults in six consecutive studies over four years. Participants began either walking or jogging -- depending on their age, fitness and preference. Twice a week, exercisers met with their groups and worked out at their own pace, but under the supervision of a group leader. Once a week, they exercised alone.

Positive feedback was crucial for exercise maintenance, Martin discovered. Participants who heard individual praise -- "Your pace is perfect right now; you're doing very nicely" -- adhered much better to workouts than those who received group praise -- "Nice going class." Adjusting daily running distance to the way people felt also paid off: 84 percent of those with flexible goals put in a third day of exercise alone each week, but only 68 percent of those with rigid goals did.

When exercising got tough, teaching people to focus on pleasant thoughts worked best for long-term adherence. Finally, if individuals were praised, it made no difference whether they concentrated on the number of miles they ran or the amount of time spent running. But without the praise, participants who thought of running distance "showed significantly poorer adherence than those assigned time goals." Teen Suicide Warning

If your children watched "Silence of the Heart," -- the program about teenage suicide broadcast last night on CBS-TV -- psychiatric experts advise you to talk with them about it.

A recent warning issued by the American Academy of Child Psychiatry and the American Psychiatric Association reports that "media coverage of suicide may increase the suicidal behavior in vulnerable youngsters."

Even when television shows broadcast warnings that the program is a dramatization, "viewers emotionally involved in the drama may miss them," the medical organizations say.

As a result, both groups recommend that parents watch the programs about teenage suicide with their children and discuss it with them afterwards. The two organizations also remind parents of the warning signs of suicide, which include:

* Changes in eating and sleep habits.

* Withdrawal from friends and family.

* Decline in the quality of schoolwork.

* Persistent boredom.

* Drug and alcohol abuse.

* Violent or rebellious behavior, including running away from home.

At increased risk for suicide: teens from families with alcohol or drug problems or where a relative has committed suicide. Suicides occur most often when a person feels that "no one needs them or no one cares," the psychiatrists say. They also tend to happen shortly after a loss, such as divorce or the death of a relative.

Nine of every 100,000 teens, 15 to 19 years old, commit suicide each year, according to the National Center for Health Statistics. That equals nearly five adolescents every day. IUD Recall Hotline

Women still wearing a controversial intrauterine device (IUD), known as the Dalkon Shield, are being urged to have the devices removed -- at the expense of the drug company that sold them.

A.H. Robins announced this week that it will even pay for examinations for women who had an IUD inserted in the early to mid-1970s, but aren't certain if it's a Dalkon Shield. Medical records may show whether a woman is wearing the device. But typically,a physical exam and sometimes an X-ray is the only sure way to tell the type of IUD.

More than 2.8 million Dalkon Shields were sold to women in the United States between 1971 and 1974 when Robins stopped distributing the device after serious health problems -- including injury, infection and death -- were linked to its use.

The American College of Obstetricians and Gynecologists warns that an IUD user who experiences any of these symptoms should consult a doctor:

* Can't find the string.

* Bad smelling discharge.

* Bleeding between periods.

* Suspect pregnancy.

For information call toll-free: 1-800-247-7220. In Virginia, call collect, (804) 257-2015.