Good advice so far. You could also try:
1) "Bottoms up squats", this is a squat where you set the bar in the bottom position (a power rack will work well), load it up and then perform squats from a dead stop from the bottom position up.
Basically it's the same thing as the pause squats mentioned above. But with the added benefit of being able to use more weight, and moving the bar from a dead stop.
I recall hearing Dan John say that these really helped him develop his front squat, don't see any reason why they couldn't be applied to back squats.
2) Isometrics. Place the pins in a power rack at your sticking point and then try to push the bar up through the pins as hard as you can. You can either do sets of 1 rep for approx. 10 seconds, or you can do sets of several reps for 2-3 seconds.