Definitely avoid doomsday stuff, and talk of zombies also immediately makes her think you're not really serious "because everyone knows that's never really going to happen." Instead focus on being prepared for natural disaster.

Hurricane Katrina was what brought my wife around... she saw how all the stores closed, supply lines dried up, and anarchy ruled for a while. She was the one who decided it would be a good idea to have enough supplies stored so that we could hunker down here for an extended period of time. Of course I'd been socking in other things for years, and she was on board with that, but Katrina pushed her over the edge so to speak to become a full partner with me in the effort. Didn't take much explaining on my part of how fragile our nationwide power infrastructure is, or how simply the transportation infrastructure can be disrupted, for her to buy into it fully.

There's enough natural disasters in the news almost daily to use that as a starting point. Begin with a casual conversation and build on it from there as she gets used to the idea. Food and water wise, we started with only stocking up on the stuff we know we use all the time, then expanded into other supplies as time went on.

Hope this helps. Great topic.