Well, I think the best you can do is put yourself in situation (ie go to your "casual dating scene") and get used to it. Also, be more confident (without becoming full of yourself) in the fact that you are attractive and that, obviously, women you meet have a good chance to be attracted to you. Once you have assimilated that fact, you're less likely to panic when a girl comes up to you.

But really, I'm taking a shot in the dark there.