One thing I have always taught my son was not to fall off to the side when completing his pitch. You see it in the pros and just about every level of ball.
I taught him to have a big stride towards the plate, finish low with head still and eyes fixed to the plate. The trailing leg come around and lands so you are squared up with the plate. This makes sure you can defend up the middle and you can move left or right because you are low with your legs under you. This also cuts the angle down so he can leve the ball that he can't of should not try to field that would be slowed or deflected so SS 2nd can't make a clean play.
As far as pop ups I generally tell him its a judgement call but generally not too far off the dirt area of the mound.
In a very intense game he chased a popup ball over by 1st baseline and ended up getting cut down by a guy that looked like a locomotive. Ended up in a tangled pile but fortunately was not hurt. In that case he probably was the best one to go get it but it could have been a disaster.