Get involved with your child's life. Do activities together. Be involved with their school and sports. Changes in your child's behavior and appearance can be signs of drug use. If a conversation lasts more than five minutes, you're probably preaching.
Kids have short attention spans.
It's better to have lots of short talks. Promote alternative activities. Encourage your child to get involved in a sport or take lessons on musical instrument. Help channel energy into positive results. Ask for help.