I think just saying "I decided I'm not drinking anymore," should be enough if they're really your friends. Honesty, and commitment to my resolution seemed to go a long way. I don't explain it anymore to anyone, just say I don't drink, and that's usually the end of it. If it were close friends I'd tell them because alcohol was killing me. I can't handle it like others. Period.
A bunch of my friends took bets on how long I'd stay stopped. They all lost.
