As I've mentioned, I've been working through the New Rules of Lifting workouts. I've discovered, through this book, a wonderful hamstring workout that also engages the glutes. You can see a video by going to the following link and scrolling down past the "Glute Ham Raise" section:

http://www.t-nation.com/readTopic.do?id=1546556

I strongly suggest giving it a try to add some variation to your hamstring exercises. You can make it harder by moving your hands in closer to your body, which engages more stabilizer muscles to keep your feet on the swiss ball.