How to motivate my wife to get fitI will write in the form of list (some of the ideas are already said), hoping that you'll find some of the ideas helpful.
1.) Motivate by Inspiring - like Ken suggested, do things that you would like her to do. People in relationship tend to mirror each other, so if she sees you doing fit activities, she will probably want to do the same.
2.) Give her physical attention - in long lasting relationships sometimes other things get into focus, so that partners forget to touch each other and show interest in each other's bodies. This is, specially for women, motivation killer. Compliment her new outfit or part of her body that you like, cherish her body, tell her how important and valuable her body is to you (among other thing), so that she finds it worth caring for.
3.) Share an active hobby - find some hobby that you both could enjoy...if she is not so much into sport, almost every women will be happy to try some dancing (salsa, tango argentino, classical dances, some other popular dances...) or hiking on interesting places, exploring nature by bikes etc...
4.) Play on the "Children card" - although it sounds a bit manipulative, I actually believe we are direct and most important role models to our children. Most children grow up having similar habits as their parent. If she does not care for herself that much, maybe she would be more motivated by doing it for the kids.
5.) Help her free some time - if she has an exhausting job, or she is overwhelmed with everyday duties, or if she does most chores in your family, maybe she is having hard time to find free time for exercise or just even for stress relief. In that case see if you can jump in and help her free some time.
6.) Tell her directly - Although weight is very sensitive subject for most people, I personally am ok with directly saying what needs to be said. For me provocation works as a motivation, but I'm aware that most people are not like this.