For me, when I asked the girl I'm with out, it was using an inside joke, actually. We had watched uhh... http://www.youtube.com/watch?v=MuOvqeABHvQ this. I sat down next to her, we had a bit of a conversation, and I used the line (from this) "Would you like to go see a movie?"
She smiled and said yes.

This wasn't the first time a silly joke actually landed me a girl friend. My freshmen year, around time for Home coming, I took a pizza box (the school sold small $2.50 pizzas) and used it as a puppet, asking her out. She said yes, with a laughing smile.

So, my advice? Make an inside joke between the two of you and use that, or do something so strangely silly she can't possibly say "no".