I don't smoke but bf used to. He recently quit thanks to my reverse psychology and my patience.
I know some guys won't quit even if their gf begs them to so I only mentioned it to him once that I wish he'll quit. When that didn't work, then I just started to buy packs of smoke and tried it out in front of him a few times (and coughing like crazy too) while telling him that I'm learning to smoke because I want to feel "closer" to him. Well, that was a total guilt trip for him because he doesn't want me to pick up on smoking so he decided to quit and be just as healthy as me.