The absolute best antiestrogen available is ovariectomy. Otherwise maintaining a low %bodyfat and a lowish fat high fiber diet with LOTS of water is the best bet. And of course avoid birth control pills like the plague! High doses of androgens may help.
There are some new anti-Es in development for premenopausal women with breast cancer. They target FSH or LH feedback rather than estrogen receptors or aromatase enzyme. But as yet these are not commercially available.