Then don't let them watch R-rated movies.
That's the suggestion from researchers at Dartmouth Medical School. According to the Washington Post:
Researchers at Dartmouth Medical School who surveyed 2,600 children ages 9 to 12 and their parents found that [the 45-percent of all] kids whose parents did not let them watch R-rated films were 40 percent less likely to consider using cigarettes or alcohol than those with more-permissive parents.Researchers admit there may be all kinds of reasons for this link -- but one of the simplest is that children want to be like the adults they admire . . . and the adults in R-rated movies are more likely to smoke and drink than those in G or PG films.