I'm not sure there's any SP legal way to lose a significant amt of weight in a month. Your body just has to decide when it is ready to let go of the fat weight. However here are some tips that may cause temporary weight loss or just make you look as if you have lost:

Eat very low sodium for several days before your trip. Remember that most of the sodium is already in foods rather than from the salt you add at the table.

Drink plenty of water, at least a gallon a day.

If you are drinking diet sodas, stop now.

Get a Pilates tape (not Denise Austin or Pilates for Dummies ) or go to a Pilates class at least 2x/wk. Pilates trains your core muscles (abs & low back) and pulls your ab muscles in, making you look smaller. It also trains you to...

Stand up straight!

Never underestimate the power of support undergarments.

Also, instead of buying a "fat dress", do you have anyone you could borrow a dress from? When I was at my "fat" weight postpartum I borrowed clothes from my mom and sister.

Jen