I've only done it from the dom side. My advice is to drop hints pretty soon, and at some point just confidently tell them that that is what you are into. Don't pussyfoot around the subject and be embarrassed, but make it very clear that you don't expect him to act upon your fantasies at all, unless he wants to.
Chances are curiosity will overtake and he'll start asking lots of questions about it, which you can use to explain more and more about the psychology and reasoning behind it and why you find your role enjoyable and why others find being dominant enjoyable. I tend to find that most people already engage in SMish activities during their "vanilla" sex, but because it doesn't involve handcuffs or other explicit memorabilia they don't class it as such. The trick is to blur the line between a good bit of rough sex and kinktastic goodness and let their own curiosity drag them down the rabbit hole.
