I smoke on and off - tend to quit for awhile then start again. Back when I was doing full-time office-style programming gigs, I found it a good excuse to go outside and walk around if I was stuck on some particular problem, and it always worked wonders in that regard.

That being said, It's a nasty/expensive habit and I don't recommend it to anyone. Anecdotal evidence: of the 600 or so people at the last startup school, I saw maybe 4 people light up, so it can't be very prevalent among good hackers.