Vitex nourishes the pituitary gland, and is good for encouraging ovulation and assisting in the luteal phase of your menstrual cycle in general. I think Vitex is an excellent place to begin when trying to balance your hormones with herbs, from what I have read.
I've been on it myself for about 2 weeks, and perhaps it will assist me in regular ovulation. I won't really know for another 3 to 6 months, as it does take some time to take effect.
I'm not exactly sure about the dosage, because everyone is different, but from the various dosage guidelines that I've read from herbal remedy books, a couple hundred mg per day is usual. I've also seen that it's generally recommended as a once a day, in the morning pill. I take one per day, 240 mg.
I think buying high quality, natural and organic herbals will suit you better in the long run, and that probably means forgoing what you usually see on grocery store shelves. Pesticides and herbicides are counterproductive to your health.
I know that Oregon's Wild Harvest sells Vitex at a reasonable price on their website, and is 100% organic. | 
