My sister used cool, slightly rumpled (but still fresh) cabbage leaves to dry her milk supply. She couldn't breastfeed so she wanted to be rid of the milk. It took about 2 weeks because she was consistent. Also, a handout I received from my OB's office says that cabbage leaves help dry the supply via some kind of chemical reaction (???). Hope this helps some!
From my experience, the leaves just worked as a relief from the pain.. like an ice pack shaped to your breast.
There are some different herbs that can help dry it up but it will eventually dry up on its own.