How do you clean dressing or mayonnaise stains off of a 100 percent polyester prom gown when instructions say do not wash and do not dry clean spot clean only?

There are spot cleaning kits available for just this job, check out your local Walgreen's or grocery store and look in the laundry section.

Of course the old standby is soda water

You could just clean it with the Dressing or Mayonnaise Stain Remover off of a 100% Polyester Prom Gown When Instructions Say Do Not Wash and Do Not Dry Clean Spot Clean Only Kit from Wal-Mart. I believe its only $19.99.