How can I become underweight?

This may sound a bit unhealthy (a lot rather), but I'm cosplaying for Anime Boston 2013 and I lost a bet and have to go as Hatune Miku(*sigh). Anyway, I have the costume and everything, and I even have the figure for it(again *sigh) I just need to drop a little bit of weight. I'm 5' 5" and weigh 135 lbs. What is a good goal for weight from there and how do I get it?
How can I become underweight?
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