This seems like a no brainer but eat as clean as possible. This means: no wheat, dairy or alcohol. I would also cut out meat, nuts and beans for digestive reasons if you want to be super strict. Try to eat foods that are natural diuretics like asparagus.
Cut out ALL salty food that is not naturally salty. For example, you could eat smoked salmon which has a naturally salty taste, but you shouldn’t be eating anything processed like salty chips or sodium-filled soups. Basically everything processed has a TON of salt so stick to only natural foods.
Drink as much water/green tea as possible (i.e. Pee as much as possible and flush everything out!)
I also recommend drinking hot lemon water a few days before and the day of (Directions: http://www.theskinnyconfidential.com/2011/08/14/hot-lemon-diuretic/) You will pee a ton and your stomach will flatten.
Incorporate HIIT workouts in to speed up your metabolism, look up some on YouTube and try for 30 minutes a day if possible. You want to get your heart rate up and tone your body as much as possible before your event.