In a link sent in by Anjan G., Victoria’s Secret model Adriana Lima explains what she does in the months prior to walking the catwalk (source).   Here’s a summary:

• For months before the show, she works out every day with a personal trainer; for the three weeks before, she works out twice a day.
• A nutritionist gives her protein shakes, vitamins and supplements to help her body cope with the work out schedule.
• She drinks a gallon of water a day.
• For the final nine days before the show, she consumes only protein shakes.
• Two days before the show, she begins drinking water at a normal rate; for the final 12 hours, she drinks no water at all.  She loses up to eight pounds during this time.