If they never ask about drug abuse, bring up the subject yourself when you see opportunities – for example, if it is being discussed at school, or there's a relevant news story. Ask them what they know about drugs and whether the subject worries them.
If your child is impulsive, you can lower their risk of acting before thinking by teaching them how to wait before making decisions – for example, counting to 10 or breathing slowly for 20 breaths first.
Start early facilitating opportunities to develop healthy outlets and social life such as sports, music, art or drama.
Encourage healthy friendships – but if your child chooses friends that worry you, you'll only intensify the relationship if you try to ban it. The best you can do (as long as they don't pose a real and present danger) is to encourage them to meet up in your home, so you can keep an eye on them, while at the same time actively promoting other healthier friendships.
Even then, however, you might not stop them trying drugs, so if you suspect this, be sure you're clear about the signs of drug abuse by asking professionals or reading up, and prepare yourself well for a difficult conversation. Practise what you'll say with a friend or partner or out loud to yourself (you could ask them to accompany you to help keep the tone steady).
Take time to settle your own fears and anger before you begin. When you speak to your child be direct, opening the conversation not with accusations but with certainty this is going to be sorted.
Finally, and whatever else, do your best to be a good role model. If you regularly drink too much or express your inability to do without alcohol or cigarettes, for example, you'll lose all credibility. Instead, exercise self-control, be moderate in your behaviour and try your best to show good judgment. This is more important than anything else you can do.