Just talk it over with her. If your in a loving relationship in and out of the bedroom, there has to be a moment where your talking about fantasies. Bring it up then, or at least hint toward it. One common mistake is trying to show her a favorite porn scene of it, don't ever do that. A lot of the porn online about it is more about Female domination (Slapping, face-sitting, whipping ect.) and that may turn her off of it. Also remember that its similar to giving head to you for her, in the fact that she isn't getting much of any direct stimulation(some double ended toys do exist but we never had a good experience with them read my Feeldoe review), this doesn't mean there is nothing in it for the 'driver' but it's a different stimulation for them. Many women enjoy turning the table, some people like the idea of giving pleasure to their partner. Don't forget that communication is going to be key here before, during, and after. Unlike when you do it to her she has to strap something on that she has no physical why to feel what she is doing in there. That's why your there to tell her what feels good and what doesn't. If you do get to the point of trying it don't just drop a harness in front of her she is the one who has to put it on and wear it so make sure she is a part of all the shopping you do for it, she will notice that your focusing on her and making her the center of the whole thing. Size, price, and comfort (For both of you) will determine what harness or set to get. If your in a good relationship with your wife she should at least want to try it once if even to check it off the list of things to do that's what happened with us just experimenting with new things and we found one we both liked so our first set was fairly cheap but served its function I reviewed that set too.
Remember! Communication is key!!!