My top three hardest habits to break: (in no particular order)
The glove slappers (I think this starts because they SAW someone else do it.)
Pointing the glove hand to first base instead of straight.
Replanting the drag foot.
I can see wanting to discourage the practice from the beginning, but why would you spend much time or energy trying to break an established leg slap? As another poster mentioned it doesn't really impact the pitch negatively and it can be a sign that they are doing other things correctly, i.e. getting the glove hand up and not swimming on the follow-thru.