First and foremost I would make sure that you let her take it at her own pace. Let her discover her love of training - be it weights or cardio, crossfit or body building. Do NOT foist it upon her or not only will she never like it, she will resent it and could end up resenting YOU for doing it to her.
Second - I would take it very slow in regards to the amount of weight being used and the effort you attempt to have her exert. Very few people get a kick out of being sore, much less so sore that even the thought of sitting down on the toilet is enough to make you give up training all together.
I dont know how (in)active your wife is, but for some people simply WALKING is "hard" exercise. I had a buddy one time that crapped out walking up an easy hill with me. He was very out of shape at the time, never exercised, piss poor diet... It hadn't even occurred to me it was possible not to walk up the hill. However, after a year or so he had lost 100 pounds and was eventually able to do 40-60 minutes on the elliptical at a very good level. He went at his own pace and accomplished a HUGE goal.
So just remember that the goal is to INCREASE her current level of activity, not to be at some arbitrary fitness level right away (Doing body weight squats and deadlifts, for example) - Maybe she needs to do "girly" push ups, 5lb shoulder presses, and body squats.
You will have to find that fine line between coaching her to do more and pushing her beyond the point where it is fun. Unfortunately, not every woman in the world is like the girls in the T-Nation videos and is willing to push a prowler until they can't move any more.
Other "Bullet Point" tips:
Focus on things like clothes fitting better and the mirror instead of her weight.
Simply eat MORE (steamed) veggies, this will lead to better nutritional habits for a number of reasons
For every week that she stays on track, do something nice for her.
BE ENCOURAGING at all times.