when trying to clean up the pull, i find it helpful to do the following:
stand straight up with the bar in your hands, shoulders back. bend the knees slightly, about 3-4 inches, do this without bending at the hip, or without leaning over. this puts you in a position that although is maybe not textbook, is a realistic representation of the position that the second pull or explosive phase actually occurs in most lifters. stop in this position, get to know it. i call this the second pull position. do a clean from this position, in fact, do several, on each one, repeat the process of standing straight, puling the shoulders together, stopping, and taking stock of where you are. then bend the knees and get into the second pull position, again, STOP, take stock of where you are, get to know this position, do another clean, repeat till you are comfortable with both positions and can get into either with little fidgeting.
now, get into the second pull position, via the method described above, stop, make sure to stop, then slowly (again, after you have stopped in the second pull position) slowly, bend at the hip so as to allow the bar to slide down your thigh till the bar is just below your kneecap. do this slowly. when the bar is right below your kneecap, resting against the patellar ligament, not the patellar tendon, but the patellar ligament directly below the kneecap, stop. stop for 3 or 4 seconds, maintainning a tight back... then slowly, slowly, extend the hip (not the knees, they should neither flex nor extend while you are going from the second pull position to the below the knee position, what i call the low hang position), keep extending the hip till you feel yourself coming to the second pull position, the position we achieved by flexing the knees. when you feel yourself in this position, simply jump, catch the bar, and viola you have a clean.
gradually repeat this exercise, maintaing the stops and the slow speed of movement on the way down, but gradually increasing speed on the way up, increasing speed to the limit of how fast you can go while still hitting the second pull position on the way up in exactly the same way as you hit it on the way down.
hope that helps.