Parenting - how to communicate with young adults in your family

I like to use this blogger to gather some tips and ideas on parenting. How can you communicate with a 20 years old son?

My son is a sophomore in a university. We have been educating him on smoking, drinking, sex and drug issues since he was a child. However, he is keeping touch those area in recent years (friends inference). We don't know how to deal with him on these issues. He kept denial about these activities, but lots of evidence showed that he has been doing these kind of activities. Thanks God he has not  got into Drug yet, but not sure he will do it in the near future...
So how can I communicate with him in a constructive way? Lecturing won't work, we have been there and it ended up lots of yelling and screaming..
Any suggestions and ideas are welcome!