Besides not being able to lock out the weight, ' depth ' is the most common reason for missing a squat in competition. The first and obvious way is to squat deeper in training and having your partner check your depth on ALL sets. Another way is to choose the correct assistance movements.
Most lifters race over to the leg press, followed by leg extensions and leg curls.
For the powerlifter I'm not so sure. Leg presses aren't all bad. What you do get from this is a large increase in ego. I suggest dropping the weight and doing very controlled movements with full range of motion. By that I mean knees to tits full range of motion.