I still kinda suck at ducking, I've hit plenty of ducking tricks, it's just not consistent eenough for me yet.
But how I practice is thus:
Set about chest to chin height, clipper set, toe set, frigidosis set, it doesn't matter. Just do that for a bit so that you know what a set that high feels like.
Once you've got that feeling down, try closing your eyes for a short period of time while the bag is peaking. It'll kinda simulate the "blindness" that ducking gives you. Try to increase the amount of blind-time. It's all about spotting the bag as it comes up. You'll find you'll start to intuitively *know* where the bag is, as opposed to seeing where it is in front of you.
Hopefully that helps, also I should practice what I preach and practice ducking.

- Kevin R.
F = G*((m1*m2)/r^2)
Know thy enemy.