Basically, your period is your body's time to slow down and rest. A restorative yoga practice is recommended at this time, with bolsters, blocks, and props. I would avoid brisk vinyasa and inversions - which are contraindicated during menstruation and pregnancy.
I personally recommend Balasana (Child's Pose), Dhanurasana (Bow Pose), Paschimottanasana (Seated Forward Bend),Baddha Konasana (Bound Angle Pose), and Savasana (Corpse Pose). You can find the postures Here
, as I am too lazy to go over all of them.
I suffered from terrible cramps when I was young. After my daughter was born, the severity decreased, but I still have them pretty bad. Eat plenty of bananas (potassium) and healthy chocolate - the kind without all the additives (magnesium/manganese). Up your calcium intake as well.
A hot bath is the best on-the-spot cure for cramps that I know of. As soon as that hot water hits, cramps are gone, and it's naptime.
If you plan on using yoga as a long-term weapon against cramps, I'd recommend buying a book or video. I really like Shiva Rea, and she has a Lunar Flow DVD
of restorative vinyasa practice (but avoid the inversion sequence). Any good yoga book should have an explanation of the healing properties of each posture. B.K.S. Iyengar's "Light on Yoga" is a good place to start. It's a thorough pictoral glossary of all yoga postures, and will tell you which poses to practice and not to practice during your period. It's also inexpensive and readily availible in most bookstores.
Hope you find something that helps! Cramps are no fun.