Lay out the strategy.
It isn't always helpful to follow-up your generalized, high level explanation with "and I know this will work, some other guys are doing it" Treat her like a business partner: show her a house, show her repair estimates and quotes, show her the comps and recent sales, show her the mortgage estimate and recent tax bills, then the market rents in the area.
This is math you should be doing anyway.
And its not like you're trying to swindle her. She can not deny the math.
If she has been on board with your stock investing (which is common gambling), then she should able to see the writing on the walls when you put pen to paper.
Also, your wife is your wife. She wants to believe in you and she wants you to succeed. It hurts her when you fail at something so her apprehension is more about that, then not thinking its a good idea. Get her far away from the idea that this might fail and she'll champion your cause as if she thought of it!
You know you've got a good thing going on this RE thing. Trust it and she'll trust it, understand it and she'll understand it. But also, show her WHY you trust it, why it makes sense. Again, She cant deny the numbers.
Spend some time putting all that down before you start the discussion, that way you're prepared. And if she has a what-if that you cant answer, there's nothing wrong with saying "Good Question, Iet me check" then coming back here and asking a guru.