THURSDAY, Nov. 15 (HealthDay News) -- The American Cancer Society launches its annual Great American Smokeout event Thursday as anti-smoking advocates push to reverse a slowdown in the decline of tobacco use in the United States.

The percentage of high-school students who smoke cigarettes has flattened over the past few years after a rapid fall in the late 1990s and early 2000s, according to the U.S. Centers for Disease Control and Prevention. Among adults, the percentage of cigarette smokers has hovered around 19 percent to 21 percent since 2004.

"We've seen a bit of a stall," said Dave Dobbins, chief operating officer of the American Legacy Foundation, an anti-smoking organization that was created by a 1999 settlement between the federal government and tobacco companies.

At the same time, though, people who do smoke are smoking less, said Thomas Glynn, director of cancer science and trends and international cancer control at the American Cancer Society.

"Today, 60 percent of smokers are smoking less than a pack a day, and it was the reverse 10 years ago: 60 percent were smoking a pack or more a day," he said. "Smoking isn't a problem that's solved ... but we've certainly made progress."

Glynn attributes the decline in the number of cigarettes smoked to efforts to boost the price of cigarettes and make it harder to find places to smoke. "They don't want to spend as much, and they don't want to stand outside in the cold to smoke," he said.

So why isn't the number of smokers continuing to dip at a rapid clip? Dobbins suspects that the intense anti-smoking efforts prompted by the 1999 settlement reached the "low-hanging fruit" -- the people easiest to persuade.

Certain groups of Americans are especially likely to smoke and may be harder to reach. The most recent CDC statistics, from 2010, found that almost a third of Native Americans smoke cigarettes, as do 21 percent of blacks. Smoking is most common among younger people and those with less education and lower incomes.

As always, it's extremely difficult to quit smoking. "The science is becoming more rigorous and shows that it's as difficult to quit as cocaine and heroin," Dobbins said.

Smoking is a difficult habit to break because it's such a routine part of people's lives, he said: "You smoke on your break, you smoke when you're eating lunch."

There are a wide variety of strategies to quit smoking, including cold turkey, counseling and medications like nicotine patches. "A combination of counseling and drug therapy will have much more success than quitting cold turkey," Dobbins said. "You'll decrease your chances if you don't look at the entire tool kit of things that can help you."

Don't give up if you relapse, Glynn said.

"What we generally find works best is earning it by going through the difficult process of trying, slipping, trying again, slipping, trying again," Glynn said. "Most smokers will take four, five, six, seven quit attempts before becoming successful."

What doesn'twork? Acupuncture and hypnosis are popular but don't work for the vast majority of people, Glynn said.

There are many online resources to help you quit smoking.

The American Cancer Society, for example, offers a cigarette cost calculatorto show how much money you're spending to smoke. It also has a guide to the benefits of quitting smokingand a quizto gauge whether you need help to quit.

More information:

For more on the hazards of smoking, visit the U.S. National Library of Medicine.