How can I lower my PMI payment?

I purchased my home two years ago. At the time I had some high credit card balances that were hurting my credit score. I had no problem getting a loan and a decent rate, but the monthly PMI payment quoted was extremely high (circumstances at the time forced me to buy the house and have to bite the bullet with the high PMI payment). I was told that my payment was high because of my credit score. Well, my credit cards have been paid down and my credit score, in turn, has increased almost 80 points to 718. Also, I have made all of my mortgage payments on time. How can I go about getting my PMI payment reduced?