This is it. Throw away the cigarettes. Bury the ashtrays. Take a long, deep breath. The Great American Smokeout -- the best day of the year for smokers to quit -- begins at midnight.

At no other time is there such widespread support for taking the courageous step of kicking the nicotine habit. Tomorrow, smokers will find encouragement not just from the American Cancer Society, sponsor of the annual event, but also from businesses, worksites and restaurants.

Some 5 million smokers went 24 hours without lighting up during last year's Smokeout. The idea is to experience being smoke-free for one day, "even if it's just to prove to themselves that they can do it," says a bulletin from the American Cancer Society.

If some smokers try it, like it and never light up again, well then, all the better. They can join the swelling ranks of former smokers -- at last count some 35 million people have kicked the habit since 1964, the date of the landmark surgeon general's report linking cigarettes to cancer.

There's no need to elaborate on all the health benefits of quitting. The risk of lung cancer. Emphysema. The higher incidence of cancer of the esophagus, pancreas, bladder and kidney. The earlier onset of menopause for women and wrinkling of the skin. Smokers know all that.

Smokers also know better than anyone else the hassles and the mess of smoking: The deep, throaty hack, the stained teeth and the tobacco breath. The smell that lingers in clothes, carpeting and drapes, anything that absorbs the billows of smoke, not to mention the cigarette burns in carpet and furniture.

They also know the cost. Not just the $1.25 a pack, but the extra time lost from work. Smokers have more colds than nonsmokers, and their children have more inner ear infections than children who live in a smoke-free home. And there is growing evidence that the nonsmoking spouses of smokers have higher rates of heart and lung diseases.

Smokers are sometimes ridiculed for not having the gumption to give up cigarettes, or face a holier-than-thou attitude from people who have quit. "When Ike Eisenhower quit smoking," says psychologist Jack Henningfield of the Addiction Research Center run by the National Institute on Drug Abuse, "he used to carry around a pack of cigarettes and offer them to people. When they took a cigarette, Eisenhower considered them a weak-willed SOB."

With the growing militancy among nonsmokers, Surgeon General C. Everett Koop recently predicted a limitation on where smokers will be allowed to light up. "The person that smokes in 1995 is going to have to smoke alone or with other smokers," Koop said. That might mean smoking in the bathroom, in the backyard or in a segregated area. "I wouldn't be surprised," he said, to see smoking limited to "out of doors."

Businesses are also jumping on the antismoking bandwagon, by restricting smoking to certain areas and sometimes banning it altogether from the worksite. One firm warned smokers that they would have to pay higher health care insurance premiums out of their own pockets if they didn't give up smoking. The company then gave smokers a year to quit, provided smoking cessation programs, and 12 months later charged a $300 fee to employes who still smoked.

Today, with the benefit of growing research on nicotine, no one suggests that it is a snap to quit -- nor do they consider kicking the habit an impossible task. The smoking field, says University of Michigan psychologist Ovide Pomerleau, "has come a long way from the time when we looked at smoking in a moralistic way."

Nicotine is a complex drug with a multitude of effects on the body, including the unique ability both to stimulate and to sedate. Research is producing new knowledge about effective ways to quit smoking -- not for a few days or a few weeks, but for good. The goal is to achieve a smoke-free society by the year 2000.

Scientists are getting new insight into withdrawal. "We're merging psychology with pharmacology," Pomerleau says. Recent ex-smokers report feeling irritable, anxious and emotional. Some say they have trouble concentrating, experience slight memory changes and worsened muscle coordination. Headaches, nausea, sleep disturbances and gastrointestinal problems are also on the list of withdrawal symptoms.

But the good news is that these symptoms, when they occur, don't last for very long -- at most a few days to two weeks. And thanks to the marriage of psychology and pharmacology, smokers now may get a doctor's prescription for a nicotine-containing chewing gum, called Nicorette, which may help ease withdrawal symptoms of some people who are severely addicted to cigarettes.

But the gum isn't the answer for everyone. People with gastrointestinal problems often find that the gum irritates their stomachs. Those with dentures usually can't chew it, and some people complain that to get the nicotine out of the gum, it is necessary to chew very hard.

Another approach is to wean smokers off nicotine by having them switch to lower-tar, lower-nicotine brands of cigarettes. But there's a catch: Many people "won't smoke these cigarettes" because they don't taste as good, Dr. Michael Russell, a smoking researcher from England, told the World Congress on Smoking Cessation recently. "And those that do smoke them tend to compensate" by smoking and inhaling more. Unless smokers continue to switch brands every week or so and consciously make an effort to inhale less, the nicotine and tar levels remain pretty much the same.

The claim is that most ex-smokers "just quit" -- cold turkey and on their own -- says the Addiction Research Center's Henningfield, who studies nicotine and other drugs. But interviews with many ex-smokers show, Henningfield says, that in fact they "actually go through a pretty elaborate routine" before kicking the habit. This includes gearing themselves psychologically for quitting, targeting a date to quit, and figuring out what to do instead of smoking.

The bottom line is hard work, willpower and a strong desire to succeed. The Excuses

Every smoker has one. Few excuses are unique. The common ones include:

*"I can't quit smoking because I'll gain weight." First off, weight gain is less harmful than smoking. "One could argue that a person could be 60 pounds overweight and still not be at the health risk of a smoker," says psychologist Pomerleau, author of "Break the Smoking Habit."

Second, smoking researchers now know why some ex-smokers put on weight and how to prevent it. Nicotine seems to suppress the desire for sweet foods, so when someone quits smoking, his or her sweet tooth often runs amok, suggest studies by psychologist Neil Grunberg of Uniformed Service University of Health Sciences in Bethesda and graduate student Deborah Bowen. Those quitting smoking should strictly monitor their consumption of sweets.

Other research from his lab indicates that people tend to become less active when they quit smoking. A conscious effort to increase exercise can help prevent the weight gain, which averages five to seven pounds for smokers and ranges from zero to 30 pounds, Grunberg says.

Women need to be particularly vigilant about what they eat after they give up cigarettes. In animal studies, females showed increases in both food and water consumption after nicotine was stopped. Males, on the other hand, ate more only if sweet foods were available. Another option for quitters: use low-calorie sugar substitutes if the urge for sweet foods is overwhelming.

*"I've tried to quit before and failed." So have many other successful ex-smokers. In fact, it takes most smokers at least three tries to break the habit, says Saul Shiffman of the Universty of Pittsburgh. The key is to keep trying, experts say. And don't be buffaloed by that the oft-quoted statement that 90 percent of smokers quit on their own. That figure is "not based on sound scientific information," says Grunberg of Uniformed Services University. "It was an estimate given by a smoking researcher roughly 20 years ago, and has been cited as fact ever since."

*"I've smoked for years, there's no point in quitting now." No reason except better health, more money and probably more time. A pack-a-day smoker spends an estimated 100 minutes a day lighting up and inhaling, and that doesn't take into account the hours spent searching for cigarettes or making late-night nicotine runs to the local store. Quit smoking and within days the body begins repairing itself. Nicotine levels diminish. Carbon monoxide clears from the blood. The lungs begin to turn from damaged black to healthy pink.

It's never too late to quit smoking, experts say. But "you shouldn't expect an immediate surge of energy," from quitting, says Shiffman, and some smokers "may have an increased cough for a while."

*"I like smoking." Chances are it's possible to learn to like not smoking just as much. The rewards will be more time, more money, more control of your life and better health. "Everybody's got to die of something." True. But no one has to die prematurely from a preventable illness. This year more than 320,000 Americans will die of smoking-related illnesses, reports the American Cancer Society. Quitting Tips

*Talk with ex-smokers for support. Studies confirm what many smokers have known all along: Those who have never smoked are far less sympathetic to a recent ex-smoker's plight than someone who has been through it (or would like to have done it).

*Warn your mate, family members, friends and colleagues that quitting smoking is imminent, and request their tolerance if you seem edgy. Ask any smoking buddies to refrain from lighting up in your presence.

*Avoid the situations where smoking is likely to occur. "The Smokeout is on a Thursday, and if you're going to participate, you shouldn't plan on going to a party Saturday night where you will be tempted to smoke again," says Shiffman, who specializes in smoking research. It may be necessary for a short time to avoid cigarette-smoking friends and relatives. People who live with smokers may need to insist that those family members refrain from smoking in their presence, at least during the early stages of quitting.

*Choose a support person -- spouse, partner, friend, colleague, relative, neighbor -- and ask him or her to help you, particularly during the early stages of smoking withdrawal. You might even want to draw up a contract with this person.

*Visit the dentist and have your teeth cleaned as you quit smoking. The absence of tobacco stains will help reinforce the benefits of not lighting up.

*Drink lots of water, get plenty of rest, exercise and try to avoid becoming overly stressed. Many people report relapsing when they are tired or under stress. But don't be discouraged if you slip and have one cigarette. The important point is to not have a second, say smoking experts, and to figure out a better way of preventing smoking again in the same situation.

Successful quitting, says a recent paper from the Annual Review of Medicine, is often associated with desire to overcome minor, smoking-related symptoms (wheezing, coughing and shortness of breath.)

What works best? The following methods were associated most with success:

*Quitting abruptly rather than slowly tapering off.

*Using a variety of coping methods to overcome withdrawal symptoms, including deep breathing, positive thinking and specific cigarette substitutes -- chewing sugarless gum, snapping rubber bands, going for a walk.

*Reliance on a variety of methods to avoid slipping back into their old habits. Successful ex-smokers avoided situations where temptations to smoke would be strong and found alternate ways to relax and cope with stress, such as new hobbies, exercise or other activities. They also took a long-range problem-solving approaches to quitting.

Many of these ex-smokers sought help and advice from others who had sucessfully quit, and they relied on support and encouragement from family (especially their spouses), friends and colleagues.