To get good at overhead squats, you need to do them often. As often as possible.
I start every training with a warm-up set of overhead squats with a broomstick.
Be sure to stretch your hip flexors, hamstrings and quadriceps. Tight hip flexors are what makes you drift forward. Lots of frequent overhead squatting makes for untight hip flexors.
And read Dan John's recommendations, if you haven't already. Squatting between the legs instead of in front.
But time will tell ... I noticed that the more weight I could front squat, the better my position became ... don't interpret this the wrong way though.