Interesting question.
I start cautiously, due to the level of anti-drug propaganda in society and the extent to which some fools believe it.
I leave myself an out. For example, I make sure when I start that I can always say "I tried smoking cannabis a few times back in college" if things begin to look hairy.
I start with cannabis, never mentioning my psychedelic use until I see how people react. If they have never used cannabis, I will probably not mention my psychedelic use, unless they seem honestly interested.
I feel that I am good at reading people, and therefore I stop talking about drugs immediately if I sense that they have some kind of strange or negative reaction.
Therefore, a few anti-drug people out there know that I have smoked cannabis, but since I am considered successful academically and socially, they (hopefully) change their opinion about cannabis more than their opinion about me.
I play down my negative experiences when talking to newbies. Once I describe my positive, life-changing experiences, though, I like to mention some difficulties I have experienced as well, so that they don't get a serious misperception.
If they sound interested in trying drugs, I generally give them a push toward, either by virtue of saying "I have tried many drugs, and I have learned a lot and enjoyed myself" or by simply showing them evidence that someone in my social position can use drugs without any obvious negative effects. (Not that my social position is THAT high, but I am a social scientist employed at a large research institution.) However, if the person seems unstable, I will push away from using drugs.
I have turned on at least 5 people who were in their 20s or 30s to cannabis using this method. I have turned two people on to mescaline-containing cactus, and one on to mushrooms, using this method. I am talking about people who have never imagined that a "good person" would use drugs, and have believed that cannabis and LSD and cocaine are all basically the same thing until I educate them. I find that saying some negative things about some drugs - such as crack and meth - tend to get them to listen more to my comments about other drugs.
One side-note about me "turning on" a reasonably large number of people: I live in Japan, where most or nearly all adults believe the incredibly stupid government propaganda about drugs (yes, stupider than that in the US or Australia, for example). Therefore, they are very surprised that I have used drugs, and they themselves have no experiences with drugs. I do not use illegal drugs while in Japan I do use those really strong drugs alcohol and caffeine), but I have had opportunities while in other countries with friends...
I also have quietly spread the word about the benefits of drugs, and my friends have used their own chances while overseas to try drugs.