Weight or Wait?

Recently, I have decided that I need to get rid of my fat.  But should I get rid of the WEIGHT now, or WAIT until after prom?  This is where the difficulty comes in... I have lost 13lbs so far this Easter, but my prom dress is now starting to sag a little.  Do I keep losing weight and get the dress taken in, and risk thedress not looking as good, or stay as I am feeling like an overweight elephant all through prom? argh! What to do?!  I just want to look like everyone else; skinny, toned but with slightly defined muscles!!! :'(