You can use an exercise ball. Lie on your back, put your heels (or one heel, depending on how advanced you are) on the ball and roll it in to you, bending your knees and lifting your butt off the ground. Slowly let it back out again. It sounds easy but it is really hard!
I have found that straight leg deadlifts, very deep walking lunges, sprints and sprints on stairs all work the hams well, though not in isolation. The ball exercise definitely isolates.

Meg