It seems as though this "incentive" topic is being covered a bunch lately. I have gone through this as well. I've been smoke free for 18 days now and really the only difficult time is when I'm at work.
I quit for her. Plain and simple. She told me how much it bugged me and that she couldn't be with someone long term who smoked. I was affraid of losing her. She stuck by me when I slipped and we cried together. I am happy that this quit is going so well. I think this time around I have found new reasons other than the main reason that keep me going.
Seeing her is like a breath of fresh air. I forget about smoking (aside from occasional thoughts). She is a major motivation for me. I enjoy making her happy, letting her know that I didn't smoke today, telling her the total number of days.
If she left I'm sure I would call her to tell her that I hadn't smoked. It seems childish, but being proud of your quit is one thing that keeps me going. I can't wait until I hit 20 days. Rolling that 10 digit is nice to see. I also purchased a home yesterday, my first. Saving money is a big motivation. My girlfriend is going to move in with me too, so being smoke free is something that will make ALL of us happy, including my family and friends, so I've found many ways to motivate myself. Using a significant other as motivation is great, but in the end if that person goes out of town or is gone for a prolonged period of time, you need to come up with your own motivation.