There is no more humbling hamstring exercise than the Glute/Ham Raise.
Seriously. Very few people can even do one complete rep. Luckily, quarter reps and half reps aren't a waste of time as even a limited range of motion on this movement will help you build the knee flexion function of the hamstrings like no other hamstring movement.
In addition to building up those hammies, it can also make an athlete virtually invulnerable to hamstring injuries as the movement lengthens the sarcomeres to an unparalleled degree.
The goal is to get down on your knees and try to lower your torso to the ground under control, then bring yourself back up. As mentioned, very few athletes can actually bring themselves up at first, so you might want to help yourself with a little arm push to get you started off the ground.
Starting position: Kneel down and lock your feet under something solid and heavy (a partner can do just fine, but he must be able to hold you down). The trunk is upright and the arms are alongside the body.
Execution: Lower yourself as slowly as possible. To do so you must produce a powerful hamstring contraction or else you'll find yourself embedded in the floor! If you're able to bring yourself back up on your own, do so, but as mentioned, most guys will need a little push-off with the arms to get moving.
An earlier, unevolved Christian Thibaudeau demonstrates the glute/ham raise.
Remember to go easy on this movement until you develop some strength and proficiency. In fact, you might want to do this movement last in your workout to ensure that your hamstrings are fully warmed (even though your strength will be at its lowest).
Try doing as many reps as possible, making sure to stop before your hamstrings rupture and you start screaming like a little girl who just sat down on a hot stove.