What really works to prevent stretch marks when pregnant?

I am sure just about everyone heard of all the urban legends and old wives tales on how to avoid stretch marks when you are pregnant by using cocoa butter, etc but what really works? I once read that to prevent stretch marks you have to think inward not outward, that zinc supplements or eating a diet high in zinc like beans will prevent stretch marks because the zinc makes the skin more elastic, do you think that will work? Have you tried anything that worked for you? My fiancée is rather petite so I know when the baby comes after the wedding stretch marks will certainly be a concern.