Dude if it's just for the one day, I strongly suggest this (incoming broscience):
2 days prior to the day, eat your usual clean lunch, then nothing but water until dinner. For dinner have a protein shake. The next day you will consume nothing with caloric value, which pretty much leaves you with water, BCAA supps (make sure no carbs), thermogenics, black tea/coffee without sugar, and you get the idea. During this day do a heap of really low weight lifting and a bit of cardio - you wanna get rid of fluid while still trying to encourage your muscles to hold onto a bit. Don't forget you're not eating anything at all this day, just non-caloric fluids. After you wake up the next morning and take a piss/whatever, take a pic of how you look in the mirror and post results.
I'm sure there's more effective ways of doing this, but I stumbled upon how effective this can be when I got my tongue pierced and had no choice lol