I have used yarrow - both as a tea and as a tincture - for well over a year now to reduce 'flooding' menstruation or heavy flow. Since I was in my early teens, my cycle has been heavy, and even more so as I got older, had kids, etc. It doesn't so much help with the pain (although it does diminish it as the blood flow diminishes) it works wonders for reducing flow. If I have to go out and am worried those first few days, I take a dropper full of tincture under my tongue or a cup of tea earlier in the day, and the problem is solved.
A note: this stuff tastes horrible (like dry grass, but even more bitter), but it works.