depends totally on the person and the situation for me.
Usually i say nothing and just dont drink, dont accept or order alcohol and figure that I dont need to explain myself to anyone and i've never been asked.
other times, in conversations, I may say that I quit drinking because it made me feel like s***, or that It makes me depressed, or that it makes me tired. (LOL! HUGE Understatements) (Try: it puts me into blackouts, I cannot control myself once I start etc..) and that I just feel so much better without it.
Another way I have put it is that I decided not to drink alcohol because there is alot of alcoholism in my family and I carry the same genes.
To my alcoholic friends, I don't hold back...