A: You want a cigarette?

B: No. I don’t smoke.

A: What? I thought you smoked.

B: I used to. But I gave it up.

A: Oh, that’s great. I’m trying to quit. I’ve tried everything…nicotine gum, nicotine patches, hypnosis...nothing seems to work. How did you stop?

B: I don’t know. I just decided to quit, and that was it.

A: Uh huh. I’m not sure I believe you.

Also:

I quit smoking.

I stopped smoking.

I gave up smoking.

I used to smoke.

I’m trying to quit smoking.

I don’t smoke anymore.