I'm new to this myself, but the thing I've been keeping in mind is good advice I've been given in all other aspects of life, so I imagine it applies here too: What will be will be.
I think we try so hard to define ourselves and give ourselves and our relationships labels. He's my husband, she's my girlfriend, I'm her lover, etc, etc, that we lose sight of how organic and fluid these things can be. Just let it happen. And talk about it. Lots. I find that when I start to think about "what it all means" with the poly thing, I sometimes start to panic imagining the cultural response to what I KNOW is the right way for me to live. Then I talk to my husband and I know that he's here with me and the rest of the world is not my concern. It's a good feeling.