What you say and what you do should give your kids a consistent message. Establishing nonsmoking rules in your household will emphasize that you're truly committed to your beliefs.
Talk to your kids about your values and expectations. Tell them you don't want them to smoke and that you'll be disappointed if they do. Explain that your rules are a way of looking after their health, just as you do with the foods they eat and the exercise they get. Be clear about the consequences that will be enforced if they do choose to smoke.
No-smoking rules should pertain to everyone, not just your kids. Ban smoking from your house and your car. If guests, friends or relatives want to smoke, send them outside.