My dad didn't have to talk about his history -- he bought pot from my friends.

He was certainly a good cautionary tale, though that's not really an option available to me. (I've never done anything beyond inhaling ambient smoke at parties.)
E.G. (hubby) and I have talked about how we'll handle it -- I think we'll both be honest, and since our stories are extremely different (me squeaky-clean, hubby... NOT) that should cover most eventualities.
But I really, really agree with the holistic part, that you don't just suddenly trot out the drug discussion from a vaccuum, but teach all kinds of underlying principles all the way through. I do the medicine/ dosage thing, too, down to vitamin supplements -- we have some gummi vites from Trader Joe's that sozlet loves, I often remind her that they're not candy, they're a kind of medicine, and she shouldn't have more than two per day.
Hubby will probably eventually impart a talk his dad gave to him (his dad was also a bad boy in his youth -- all of his side of the family was, and I SO hope sozlet takes after me in that one respect). I forget the details, but it obviously made a big impression on E.G. It was something like, do whatever you want, as long as your grades are good, you do your chores, and you're respectful to your mother.