My GF told me, while we were still dating, that she liked me and all, but if we ever were to go further than dating, i had to give up smoking. i told her, fine, but i want something in return then. An eye for an eye so to say. I have to give up something i like, and so does she. What we agreed on is not appropriate on MMO-C, but im sure you can think of something she finds anoying about you, and tell her, that youll stop that, if she doesnt start smoking.
EDIT: To make my point more clear: Its not appropriate to try and change your partner, without allowing her to change a little on yourself as well. We all have little flaws, our SO finds anoying/unattractive. While it may not be as much a deal-breaker as what you describes smoking to be, it may be something worth of notice. And sacrificing something you care about for her better, also shows her that you really do care about her, and aint just up to changing her, but willing to change yourself as well, for the better.