The following are all taken directly from Coach Joe Brockoff and his Super 8 Hitting System.
Misconception #1 - Swing Level - The fact is that one of the
worst mistakes a hitter can make is trying to keep the bat level
as he goes to the pitch.
#2 - Get The Back Elbow Up - If we had to throw a punch,
would we have the elbow up? If so, it would NOT be an
effective punch. The elbow needs to support the hands.
#3 - Stay Back - The proper term here is not to stay back,
but to START BACK. We collect our weight on the backside.
#4 - Squish The Bug - The back foot should not "spin" on the ball
of the foot, the back foot should roll up on the back toe as the
hips rotate toward the pitch.
#5 - Roll The Wrist - The correct way is when a hitter is in contact
with the ball, his top hand is in adduction (like a hammer) under the
bat, and it stays under the bat through extension. Does wrist roll occur?
Yes, when the bat is coming out of extension and is coming around to
the opposite shoulder at the completion of the stroke.
#6 - Get The Hands Away From The Body - This is incorrect! The
fact is that the hands require body support during the hitting stroke.
The farther the hands get away from the body, the weaker the stroke.
#7 - Stride In The Direction Of The Pitch - Stride in the same place
every time, directly ahead, with a 6 inch glide step. It is the
ROTATION of the hips, not the stride, that determines hitting the
inside and outside pitches.
The objective in hitting is to be quick with a compact stroke, making
full use of the body action to support the stroke.