When I am looking at a guy that I'm in to and he looks at me, how do I smile?

As this is an intimidating situation to me, I always end up with a deer-in-the-headlights look, which does ZERO to indicate that I am interested... or else I muster up some HORRIBLE fake smile and make him think I am some sort of sociopath.

How can I learn to just have a nice, appealing, natural smile when this happens?