A few pointers courtesy of Tommy Kono's book (just paraphrased, not quoted):
Get a stick long enough for you to take an overhead squat grip on it. Hold it with straight arms in front of your hips. Grip it tightly and, with locked arms, lift it from there overhead and lower it behind you until the stick rests against your buttocks. Then it bring it back overhead and back around to your hips. Repeat. Take a wide grip here, wide enough that it doesn't hurt. Cut or draw marks where you take your grip on the stick and strive to narrow them.
You know the exercise. While you're doing it, though, concentrate on a couple things to make it easier:
Set a wicked arch in your back as you're descending into the squat, and maintain it as long as possible.
Punch your shoulders up 'into your ears'
Rotate your wrists outward, so that the crook of your elbow is facing aggresively away from you, as you descend into the squat.