Summarize this article:

Sage contains a natural estrogen that is reputed to dry up your milk supply. You can find sage in one of two forms:  As a tea: buy sage tea at your local specialty health store, and brew it along with some milk and honey. As a tincture: buy a sage tincture, which is mixed with a little bit of alcohol, at a health food store. This tincture is reputed to be a bit more efficient than sage tea at drying up your milk supply. Cabbage leaves work great as they are cool, and there is a component to them that naturally dries up your milk.  Apply them to your entire breast and replace them when they wilt. Vitamin B6 is reputed to stop the body's production of plasma prolactin, which is what causes mothers to produce breast milk. Several studies, however, found no statistically relevant data to suggest that vitamin B6 actually helped women suppress lactation.

Summary:
Drink sage tea. Apply cold compresses or cabbage to your breasts. Take vitamin B6.