Cathie Jung wore her first corset for her wedding in 1959. Now she wears it 24/7, and when it becomes loose fitting, she would wear a smaller one. The result? She now has a 15-inch waist. Link to Cathie's website (take a look at the X-ray pics!)
Notably, she doesn't have the world record on smallest waist ever. This belongs to Ethel Granger (13-inch). Link to Ethel's website.