Be the first to say something. If you haven't talked to your preteen about drugs, smoking or anything else they could be facing because they haven't had to face that problem yet, talk to them now! Don't avoid it until it becomes a problem or you start to see "signs". Be proactive with your preteen.
Role play. Let your preteen be the one who offers you a cigarette. This will be an eye opening experience. Say no and keep saying no. When you preteen says, "I couldn't say that", ask them what they could say or do. Then role play with your preteen saying no. Practice until your preteen feels comfortable enough to do it on his own with his friends. Learn how to role play here.
Being rude is sometimes okay. Let your teenager know it is okay to avoid people who are trying to get him to do something he does not want to do, even if it is an old friend.
Let them make you the scape-goat. Tell your preteen that there is nothing wrong with using you as an excuse. Saying, "My mom would be so mad!" to a friend who is trying to get them to smoke is a perfectly good enough excuse to get out of the situation.
Be available. Be ready and available should they need to come to you with questions or thoughts on a situation. Even if your teen didn't make the right choices, you can help them come up with a better solution the next time the situation arises.