Don't explode the weight up at the beginning of the rep. This leads to injury. Start the rep slowly and under full control. Accelerate after the first one-third or so of the movement.
Flex your feet. Bring your toes up toward your body. This takes the calves out of the movement causing the hamstrings to have to work harder.
Turn your toes in to work more of the inner portion of the hamstring. Turn your toes out to target the outer portion.
Use the Gironda leg curl to hit the upper hamstrings and glutes. Just lift the upper body off the pad.