Lots of woman advice here lately.
Play it cool. Act like you don't care. Don't ignore her, just act secure, like you know you are the best thing going and she will realize it soon enough. Its not game-playing, it is anti-game.
Think about it this way:
If she's still hot for the ex, nothing you can do will change that. However, if you play it off, one of 3 things will happan...
A) she is so bewildered by the fact that you aren't bending over backwards for her attention, she will short circuit and have to persue you out of sheer curiosity.
B) she will be so amazed/impressed by your show of security and confidence that she will be all the more attracted to you.
C) you leave the cage door open, and this bird flies away, unfortunate maybe, but you've invested the least amount of time possible, or more importantly, the least amount of your heart. That way, you know it wasn't meant to be, and you aren't crushed.
I used this same strategy with my wife before we were married. We'd been dating a few months, then she hears from an old (potential) boyfriend and wanted to know one way or another if the spark was still there. I not only didn't object, I drove her to the airport so she could visit him out of state! She spent a couple days there, and flew back as soon as she could, knowing now that this poor shmoe in Atlanta couldn't hold a candle to me.