My wife didn't really care about mileage when we first got our Escape. She wasn't doing terrible, but after seeing what I could get from it, she became interested. Now she strives to squeeze more miles out of each gallon. One of the biggest helpers for her is the Ultragauge showing the instant mileage. She even told me that she wants a Scangauge so she can see what is going on with the hybrid stuff. The best part is that I'll probably have an easier time convincing her to let me do some aeromods to it.
One thing you could do is show her the difference between your tanks or trips and hers. Even better would be showing it as cost per mile (or 10 or 20 miles). One of my main motivations for hypermiling is saving money, its hard to argue with several hundred dollars saved every 10,000 miles.
In the end, though, it all comes down to how far you're willing to push it.